Classic British SparesKlempf British PartsBaxter CycleBritBike Sponsor SteadfastCyclesSRM EngineeringLucas Classic MotorcycleHepolite PistonsIndustrial tec supply

Upgrade your membership to: Premium Membership | Gold Membership | Life Membership | Vendor Membership | Site Sponsor Membership
Welcome to BritBike Forum!
Britbike forum logo
Member Spotlight
Gordon Gray
Gordon Gray
North Carolina
Posts: 4,508
Joined: August 2001
ShoutChat
Comment Guidelines: Do post respectful and insightful comments. Don't flame, hate, spam.
Search eBay for motorcycle parts in following countries
Australia, Canada, France, Holland, Italy, United Kingdom, USA
Previous Thread
Next Thread
Print Thread
Page 1 of 6 1 2 3 4 5 6
#843806 03/22/21 10:26 pm
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Hi all.

I've had my 1966 BSA C15 for a few years now. It suffers with cough and stall syndrome at low rpm. I've read on several forums that this is common on these bikes, but I've never managed to find out the cause, and better still, the cure.

It's only a problem when having to stop in traffic, but with the type of riding I do, that's quite often. Tickover is a bit random as well. Tends to fluctuate up and down.

Maybe someone on here will know more about this apparently common fault, and would be good enough to enlighten me🙂

Cheers,
Gary.

Support Your #1 BritBike Forum!

Check out British motorcycles for sale: British Motorcycles on e-Bay UK, British motorcycles on e-Bay North America
Joined: Aug 2001
Posts: 7,127
Likes: 76
Britbike forum member
Offline
Britbike forum member
Joined: Aug 2001
Posts: 7,127
Likes: 76
Originally Posted by Douglas Clark
I've had my 1966 BSA C15 for a few years now. It suffers with cough and stall syndrome at low rpm. I've read on several forums that this is common on these bikes, but I've never managed to find out the cause, and better still, the cure.


Gary,

Let's start with what exact carburetor do you have installed, as well as your ignition system, points -EI, battery or battery-less, etc.?


Jon W.


1957 6T Thunderbird 650
1968 T100R Daytona 500
1971 TR6R Tiger 650
1970 BSA A65F 650
1955 Tiger 100 - Project
1971 BSA A65 650 - Project
1972 Norton Commando 750 "Combat"


"Every time I listen to AC/DC, so do my neighbors"

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Hi Jon.

Looks like an original AMAL Monobloc carb.

Standard points ignition.

Running with battery as standard.

I did read about one guy that cured his cough and stall by fitting an OKO carb. Very cheap from China. Would rather keep original if possible.

Joined: Mar 2005
Posts: 10,638
Likes: 64
J
Britbike forum member
Offline
Britbike forum member
J
Joined: Mar 2005
Posts: 10,638
Likes: 64
Surging, or what is often called hunting, is caused by a lean condition.

Cough and stall is often caused by a lean contion.

Force the mixture at idle, and very low throttle opening, by turning in th epilot air screw making the low throttle opening mixture richer.

If it improves the situation it would cause me to check slide cutaway.
HTH

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Thanks for that John.

I will try that tomorrow hopefully.

The slide cutaway is the standard size, but on an older carb like this I guess it may need tweaking.

Cheers.

Joined: Dec 2004
Posts: 11,177
Likes: 114
Britbike forum member
Offline
Britbike forum member
Joined: Dec 2004
Posts: 11,177
Likes: 114
And the random idle is normally a worn slide finding different positions to close on varying the air leaking past the worn slide, which won't be helping the cough.

You can have your carb and slide resleeved back to std once you confirm the slide is correct on its cutaway.

Kevin Traill at Alverstoke Restorations in Gosport, Hampshire.
His details are: [email protected]
Telephone: 02392 580708 (after 16:30).

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Thanks Kommando.

Good point regarding the worn slide. I will measure it up, and the tube, to see if there is any wear. I'm sure there will be some figures in the Manual for me to go by.

Thanks for the alverstoke restorations info. I will call him and have a chat once I know a little more.

I'll try the pilot air screw thing tomorrow and take it from there.

Joined: Aug 2001
Posts: 5,741
Likes: 139
G
Britbike forum member
Offline
Britbike forum member
G
Joined: Aug 2001
Posts: 5,741
Likes: 139
there are no figures in the Manual to gauge slide wear. try this, remove the air cleaner, start the bike, put a finger against the slide, does the tick over change, does it stop rattling , if the answer to either of these is yes, the slide is worn.


71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Thanks Gavin. I'll give that a try.

Lots to do tomorrow!

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Finally had a chance to screw the mixture screw in a little. It's now just over 1/2 a turn out, which is clearly not ideal, but has served a purpose.

Doing this has made a surprising difference. I have been able to achieve a much more even and steady tick over (not quite perfect yet), and the cough and stall has improved a little as well.

Regarding Gavin's idea, when I tried to push the slide with my finger, the tickover changed before I even reached the slide, no doubt due to my hand blocking half of the intake! So I got myself a little stick and pushed against the slide. Each time I did this the tick over slowed down. It also became more steady as it slowed.

So it looks as though I need to get the carb off and get it either fixed or replaced.

Joined: May 2013
Posts: 7,926
Likes: 132
Britbike forum member
Offline
Britbike forum member
Joined: May 2013
Posts: 7,926
Likes: 132
Copied from a thread on the triumph board, I think John described your issue perfectly....

Originally Posted by John Healy
If you have a "worn out" carburetor it usually refers to the slide bore is worn oversize. Most the wear typically appears at the bottom edge of the slide and adjacent area in the body. This lets more air enter the intake manifold reducing the vacuum signal on the fuel metering systems. The most affected period is at very low throttle openings.

Turning the air mixture screw out 1/4 to 1/2 turn further reduces the vacuum. It is as if the body was worn more. Turning the air mixture screw in a 1/4 to 1/2 turn increases the vacuum signal. This compensates for the lack of the vacuum lost by the worn slide/body.

Years ago AMAL increased the clearance on the slide for safety reasons. It help prevent the slide from sticking when the mounting hardware is over tightened. To do this they had to make changes to compensate for the loss in the vacuum signal. They did this by making changes to the body and the related bits.

If this clearance isn't maintained when sleeving the carb it throws all of the low throttle opening jetting askew. To get the low speed jetting to work as designed you are looking for around .0035" clearance. If less, corrections have to be made and you have to be very careful when tightening the mounting hardware.

For design/engineering reasons, if you choose to use a Stay-up float, you should always use it with an aluminum needle. Otherwise the old white float and brass needle still work as designed.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
I agree Allan, it does indeed sound similar. Thanks for digging that out for me👍

My only concern with that explanation is the clearance figure given. 35 thou is quite enormous in terms of clearance. 0.89mm in fact. I wonder if that really is correct? I would imagine it would rattle around like a good'un with almost a millimetre clearance.

I understand the logic regarding body distortion as I've done that myself with a Concentric AMAL.

Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
Douglas, the figure is ".0035" clearance" which is 3.5 thou.

Joined: Mar 2005
Posts: 10,638
Likes: 64
J
Britbike forum member
Offline
Britbike forum member
J
Joined: Mar 2005
Posts: 10,638
Likes: 64
Mr Clark: As Koan58 said it is .0035 inches or the thickness of a average human hair, or in that funny way of measuring things, .0889 watchamacallits (or millimeters if you so choose - It seems I don't).

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Oops, sorry guys. Being a bit dim. It is of course 3.5 thou

I will strip the carb tomorrow and see what I find.

Many thanks.

Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
That's only the original designed clearance for an AMAL Concentric. Any carb that has done 10's of thousands of miles will have more clearance, to an extent depending largely on whether it has ever been run without a decent air filter.

However "Looks like an original AMAL Monobloc carb" from your earlier post makes me doubt all of the previous stuff.
Sorry, only just noticed this. Mono's were made from the same poxy wearing metal. I don't know if the same clearances apply to them as with the concs. Is it important to you to keep an original carb?

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
At the moment I'm inclined to stick with the Monobloc carb. If I can improve on this cough/stall business just a little I'll be quite happy. There is a little play in the slide moving it front to rear (around 10 thou), but much less side to side movement.

I should also say that the choke slide that normally sits inside the throttle slide is absent. Would this make any difference?

The current air filter is an original type with a few discs of steel mesh and a piece of thin cloth in the middle. I have washed this out with petrol so it should be clean ish. Obviously I have no idea what types of filter have been fitted during its life.

I fixed the AAU yesterday as I found it was doing precisely nothing. With the weights fully extended outwards the springs were still loose. I bent the spring mounting posts so that the slack in the springs was taken up. This has improved mid range a good bit and has added a little to the higher rpm range as well. So I'm happy with the general performance.

Tomorrow I will run the bike with the air mixture screw at 1/2 turn out, and I've moved the slide needle up a notch, as the AMAL info on their website says position 2 is correct. Mine was on position 3.

I would however be interested in an alternative carb suggestion if you have one. If I could be persuaded of an easily swapped alternative I might just be tempted!!

Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
I would guess a Concentric 926. Just a guess, folk here will have more experience.

Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
If the choke isn't connected, it is important that he hole in the top is blocked in some way, otherwise it allows leakage of air to the venturi under the slide.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
I did wonder about that open hole when I saw it. I will look into a way of blocking it off. With that blocked I may be able to wind the mixture screw back to its normal position.

I will certainly consider the 900 series carb if I cannot get this one working better. I have a spare 626 from my Bantams, do you think that might work?

Although isn't the C15 inlet 24mm?

Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
Sorry Doug, I typed without thinking enough. It would have to be 600 series for that sort of bore in Concentrics. I believe new Monoblocs have been available in recent times, but a quick search shows them out of stock:
http://amalcarb.co.uk/Monobloc-series/375-series/base-line-specifications.html

I'd just plug the choke hole in a simple crude way (putty or similar) and go from there.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
I have some thin copper sheet. I'll cut out a section to cover the hole and glue it into place making sure I copper or glue overhangs the edge. I'll use a fuel resistant glue of course.

If none of this helps, I'll look into jetting up my spare Bantam 626 to suit. The pilot is fixed, but it's a 25 pilot anyway, same as the C15.

Joined: May 2013
Posts: 7,926
Likes: 132
Britbike forum member
Offline
Britbike forum member
Joined: May 2013
Posts: 7,926
Likes: 132
The problem with the bantam carb is that it will be fitted with 2-stroke jetting and spray tube. If you have the 4-stroke versions to fit in its place then it can work... but if you dont you will find jetting can be quite difficult.

As for using a 626 carb, you shouldn’t find any problems in doing this as far as the increased Venturi size from the carb. You could always get a 24mm Tufnol spacer and taper the opening to 26mm on one side.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Hi Allan.

The carb is from a Bantam B175, and is therefore partially 4 stroke anyway. The jet holder, the needle jet and a couple of other bits that I forget are already 4 stroke items.

Hopefully I'll get a little improvement from the Monobloc, then I won't need to use the 626.

Joined: May 2013
Posts: 7,926
Likes: 132
Britbike forum member
Offline
Britbike forum member
Joined: May 2013
Posts: 7,926
Likes: 132
Is the spray tube angled or cut square?


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Spray tube is angled.

Joined: May 2013
Posts: 7,926
Likes: 132
Britbike forum member
Offline
Britbike forum member
Joined: May 2013
Posts: 7,926
Likes: 132
Originally Posted by Douglas Clark
Spray tube is angled.

This is the 2-stroke type. You want to squire a squared off one, carefully tap the existing one out and carefully tap the new one into place. Taking note that the spray tube and it’s recess have a tooth like appearance. So pressing it by feel into position with a drift of the right diameter is required.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Thanks for the extra bit of advice Allan. I've not changed a spray tube before, so nice to get some tips👍

Joined: May 2013
Posts: 7,926
Likes: 132
Britbike forum member
Offline
Britbike forum member
Joined: May 2013
Posts: 7,926
Likes: 132
Originally Posted by Douglas Clark
Thanks for the extra bit of advice Allan. I've not changed a spray tube before, so nice to get some tips👍

No problem Douglas, just be careful not to damage the visible part of the tube. It is after all a jet of sorts, as its shape determines the effective vacuum draw on the needle jet as you transition from slide to needle jet positions.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Will do Allan, thanks👍

1 member likes this: Allan G
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Just a quick update as I don't like to leave these threads unfinished.

I got side tracked into wanting to sort out the timing before going any further with the carb. Long story short, I ended up fitting a Boyer EI unit, which so far works great. It has made quite a dramatic improvement to the general running, lovely mid range now.

However, I still have the cough and stall at low rpm. It improves by screwing the mixture in, but even with it fully in it doesn't cure it. The idle is still erratic.

A number of years ago I fiddled with my Bantam carb for a few weeks, but never got it quite right, so I bought a new one and it's been good ever since, so I have today purchased a new AMAL Monobloc in the hope that it will cure my issues. I will update next week when I have fitted and tested it.

On a related issue, I noticed today that the inlet on my barrel is extremely tough. I think it is unlikely it left the factory like that. I've posted a link to a picture below. Not done that before so hope it works!

https://www.dropbox.com/s/tckxk3x3b4b9yfu/2021-04-08%2012.31.53.jpg?dl=0

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
So the new carb is fitted, but the cough and stall plus the yo-yo tickover remain.

The fitting of the EI and new carb have totally transformed the bike in every other way, and has made the bike a pleasure to ride, but I'm now more keen than ever to sort out the two issues that I originally set out to cure, the tickover and the cough and stall.

I probably should have mentioned this before. My plug, after just 30 seconds of tickover becomes black and sooty. After a couple more minutes of ticking over it becomes thick with deep black soot. I'm now suspecting that I'm getting oil into the combustion chamber somehow to cause sooty blackening this quickly.

While riding today I pulled the plug cap at about 4,000 rpm (a guess). The plug was showing on the lean side, but not badly. A light grey/brown. Once home I let the engine tickover for a minute, and the plug was sooty again.

If oil was occasionally dripping into the combustion chamber from a valve guide/seal, I guess this could cause the tickover to fluctuate, blacken a plug at tickover, but what about the cough and stall?

Very little in the way of smoke from the exhaust, even at tickover.

Anyone any thoughts on this?

Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
Hi Doug,
Is that pic of the top of the cylinder? So you’ve recently removed the head?

If that is what it is, then it does look dreadful (though pics on the net can be deceptive, it appears to be have been done with a half-round file!). It looks extremely coarse and nothing like a proper honing pattern.
If it truly is that rough, god knows what its doing to your rings and piston.

While such a surface could allow more oil than usual into the combustion chamber, I’m doubtful that this is the main cause of your dry sooting at idle. Oil usually results in “oiliness”, not dry soot.

This is more likely due to the idling mixture being set too rich, possibly when the engine isn’t fully warmed up (say at least 5 miles riding), especially as the plug seems to be pretty good at 4000 rpm (presumably somewhere in the mid-throttle range).

Note that the recommended idle air adjustment is 2.5 turns out on a Monobloc, not the 1.5 turns of the Concentric.

Have you strobed the Boyer ignition? Until that is done, carb tuning will be “pointless” (a silly jest). But that is the order in which to do things.

Good luck.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Hi Koan.

Sorry if I confused you, but that picture is of the inlet. I've never taken the head off as yet. I've had the carb off a number of times, but only noticed how rough the inlet was the other day. If I poke my finger down the inlet, the roughness continues all the way to the bore, but I presume not into the bore itself! It doesn't even feel normal. It has the feeling of an alloy rather than steel. Anyway, the way the bike is going right now, I don't think it is causing a major problem.

I did wonder about the dry soot as opposed to oilyness, so thanks for clearing that up. I was already planning pulling the engine apart!

I've been trying to look up the air screw setting unsuccessfully, so I've set it at 1.25 turns out. I had no idea the Monobloc starting point was 2.5 turns out, so thanks for that👍 I'll reset it first thing tomorrow and see what difference it makes.

"Pointless", I like that🙂,
Yes, the Boyer ignition has been strobed. It only took a couple of minor adjustments from the Boyer "starting point" marks to get it right. It was quite a pleasure to see the mark move as the revs rose to being exactly in line with my 33.5° mark and then stop. Very pleasing.

Yes, the plug cap was pulled at around mid throttle, perhaps a tad more. A little hard to judge when you're hanging off the side of the bike.

I think changing the mixture screw tomorrow will be interesting. I'll update here with any news.

Thanks for your informative answer.

Joined: Aug 2015
Posts: 1,755
Likes: 37
A
Britbike forum member
Offline
Britbike forum member
A
Joined: Aug 2015
Posts: 1,755
Likes: 37
I interpreted that photo as showing the inlet port, if so it is the ideal surface finish


BSA B31 500 "Stargazer"
Greeves 200 "Blue Meanie"
Greeves 350
Greeves 360
Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500 sprint/LSR bike "Deofol"
Jawa 500 "Llareggub"
Aprilia RSV Mille "Lo Stregone"
'35 & '36 OK Supreme
Kawasaki ZZR1400 "Kuro no senshi"
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Hi Andy.

That's interesting. I can't claim to have made a study of inlets, but certainly my Bantam ones are a lot smoother. Maybe because they are 2 strokes?

Thanks for alleviating any concerns I had about it👍

Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
“I interpreted that photo as showing the inlet port, if so it is the ideal surface finish”

For many years the prevailing tuning technology was to “port and polish”.

There is a more recent school of thought which tends toward smaller ports for increased gas speed, and a rougher port surface to promote turbulence in the outer flow region.
This makes sense perhaps, more so for a racer, but as far as I know no production engine had ports dressed in that way.
That is something the PO has done, perhaps for the turbulence reason (on a C15?) or to fit a slightly larger carb to the manifold.

Is the hole still the same diameter as the carb throat?
The rough edges look as if the inlet has been filed somewhat.

This shouldn’t matter much as long as a good seal can still be made between carb and flange.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
The carb throat and the inlet width are almost identical in size. Can't remember the measurements now, but something like a few thou difference. I can't even remember which was the biggest now. So they are both essentially 7/8" (22.22mm).

When I first touched it, I instantly thought it was some sort of flexi alloy liner being used as a reducer back down to standard size. I believe some of the other C15s had a slightly larger inlet, so maybe the head from one of them, but I'm not so sure now. As you suggested, possibly just roughed up by a PO for the extra tiny bit of power?

I am definitely from the era when polished ports were all the rage. I even polished my Suzuki AP50 ports! I was unaware the tide had turned back to rougher ports.

koan58 #845860 04/12/21 11:22 am
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
"Note that the recommended idle air adjustment is 2.5 turns out on a Monobloc, not the 1.5 turns of the Concentric."


An interesting morning.

I turned the air screw out to 2.5 turns as Koan suggested. I then warmed the engine thoroughly. At 2.5 turns out I have been able to turn the tickover down much lower than before. As I turned it down it became much more steady until I reached a point where it was happily thumping away with no yo-yo whatsoever. Very pleased with that.

The only problem with this, is that it has accentuated the cough and stall. It's now much more difficult to open the throttle without it cutting out, even when opening it gently, whereas before I could at least open it gently and get away with it. If I raise the tickover, the cough and stall improves, but then the tickover starts to yo-yo again.

Another point is that after 10 to 15 minutes of tickover, the plug is nowhere near as sooty as before. In fact it's not sooty at all, just black. I put the plug back in and then ran the engine at maybe 2,000 rpm for a short while and cut the engine. The plug was then heading back to a browner colour, although still quite dark.

Beginning to wonder if this cough and stall business is going to be with me forever!

Joined: Aug 2001
Posts: 5,741
Likes: 139
G
Britbike forum member
Offline
Britbike forum member
G
Joined: Aug 2001
Posts: 5,741
Likes: 139
Try a slide with the next size smaller cutaway, if it has a 3 .5 fitted , try a 3.


71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Joined: Dec 2004
Posts: 11,177
Likes: 114
Britbike forum member
Offline
Britbike forum member
Joined: Dec 2004
Posts: 11,177
Likes: 114
This is where a gunson colortune can come in useful, it shows the colour of the combustion inside the chamber using a glass window, the right idle colour is Bunsen burner blue but as you start to open the throttle you want a momentary rich mixture with yellow showing, if its blue then you need a slide with a richer cutaway. Float height may be an issue to check first as fixing that is a lot cheaper than a new slide.

1 member likes this: Allan G
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
I've just checked the float height and it seems to be correct, although I had a bit of trouble with air locks in the tube I used. I ran a tube from the pilot jet around to the float bowl cover and taped the tube to the cover. When I switched the fuel on, the bottom of the tube fills up but then fuel flow stops with a 2" air lock. I had to use the tickler to get it going again, let the fuel flow past the dot on float chamber cover and then wait for it to go down. It would stop about 1/32" above the float height marker.

I understand the logic of trying a richer cutaway, but wouid find it slightly annoying after having bought a new carb with standard settings, although I have to admit that I am running a slightly richer cutaway on my Bantam, and I believe this is necessary due to the ethanol in our fuel nowadays. I will order one up tomorrow.

A friend has a colourtune that I can borrow, so I'll give that a go tomorrow first thing.

Thanks for all the help and advice I'm getting here. Much appreciated.

Joined: Aug 2001
Posts: 5,741
Likes: 139
G
Britbike forum member
Offline
Britbike forum member
G
Joined: Aug 2001
Posts: 5,741
Likes: 139
Its possible it may be too rich on the cut away, if its too lean the cough has a spit back through the carbs more like a sneeze, if its too rich it may be OK under load, if it doesnt accelerate and just dies, its too lean, make sure the wee holes in the floor of the venturi are clear , they are whats happening delivering fuel just after closed throttle when it coughs, if one is blocked it might just be that. Is your fuel supply clean?
Its fair to say someone has been fiddling with the port, between that and modern fuel , different pipes and filters maybe STD doesnt always work.

Last edited by gavin eisler; 04/12/21 11:05 pm.

71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Hi Gavin. Thanks for your input.

After a cough and stall, there is often (90% of the time) a sneeze through the carb, so that does sound lean. A sneeze is a great description. Just what it sounds like.

It's a brand new carb, so one would assume the tiny wee holes are clear, but I do appreciate that occassionally they may not be, even on a new carb. However, given that the tickover is now very nice, I would think they are clear. I don't doubt the carb will be off again some time very soon, so I will send a jet of carb cleaner up the little hole at the rear to make sure.

Fuel supply is new and clean, but to be frank, this cough and stall has been happening for years on many different tanks of fuel. Only just recently decided to try and fix it.

I'm keen to try your suggestion of a richer slide. I'm actually going to get the next two sizes to make sure!

A friend has also suggested I re-check my timing, although I'm pretty convinced it's spot on. This feels like a carb issue to me.

Joined: Aug 2001
Posts: 5,741
Likes: 139
G
Britbike forum member
Offline
Britbike forum member
G
Joined: Aug 2001
Posts: 5,741
Likes: 139
90 percent of carb faults are electric
The slide is worth a try if you are happy with the sparks.is it the original ignition switch?

Last edited by gavin eisler; 04/12/21 11:50 pm.

71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
I'd like a £ for every time I've heard that........and another £ for every time it turns out to be true!!😂

Joined: May 2013
Posts: 7,926
Likes: 132
Britbike forum member
Offline
Britbike forum member
Joined: May 2013
Posts: 7,926
Likes: 132
Originally Posted by Douglas Clark
A friend has also suggested I re-check my timing, although I'm pretty convinced it's spot on. This feels like a carb issue to me.

Many timing lights have an advance dial on them, depending on the model you have depends on how easy it is to knock.

When you stobe time the bike just check that the dial is turned all the way anti-clockwise (turned back if you prefer). The one on mine is quite free to turn (I have the chrome one with the rpm readout etc) I used to have hell of a job timing it up until I got some duck tape and fixed it in place.

Last edited by Allan G; 04/13/21 9:20 am. Reason: Didn’t want to give incorrect info

Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Thanks for that Alan, but no dial of any sort on my Gunson timing light. I must have the basic model!!

Joined: May 2013
Posts: 7,926
Likes: 132
Britbike forum member
Offline
Britbike forum member
Joined: May 2013
Posts: 7,926
Likes: 132
Originally Posted by Douglas Clark
Thanks for that Alan, but no dial of any sort on my Gunson timing light. I must have the basic model!!

No worries, ‘twas just a thought.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Joined: Dec 2004
Posts: 11,177
Likes: 114
Britbike forum member
Offline
Britbike forum member
Joined: Dec 2004
Posts: 11,177
Likes: 114
Originally Posted by Douglas Clark
It's a brand new carb, so one would assume the tiny wee holes are clear, but I do appreciate that occasionally they may not be, even on a new carb. .

A few years ago it was a very frequent problem with swarf being left in small galleries on the new Concentric's, so worth a look. However if the same cough has been there for years then its likely the wrong cutaway.

The Tiger cub has a similar issue, I cured this by fitting an 850 Commando slanted spray tube combined with the special shorter needle, this was on a 600 series Concentric carb so would not carry over to a Monobloc without some testing and adaption.

The origins of the 850 slanted spray tube and shorter needle was a noise test which required the bike to be moving slowly in 4th and the throttle opened sharply and noise levels taken as the bike accelerated. With the std spray tube and needle the 850 did not accelerate at all and so did not pass the test and AMAL found the solution. The theory is the slanted spay tube retains a reservoir of fuel rich air that's taken up on fast opening of the throttle giving a momentary mixture enrichment for the acceleration to kick in.

1 member likes this: Allan G
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Interesting story Kommando.

I wonder if all this testing was done outside, or whether they had the luxury of a rolling road to make testing easier?

Does that slanted spray tube differ from the other slanted ones found in AMAL carbs?

I'm going to check my timing today or tomorrow depending on what demands are placed on me in the next half hour, but I'm still of the mind that's it's carb related, so have ordered the next size cutaway (3.5), so hopefully that'll be here in a few days.

Joined: Aug 2001
Posts: 5,741
Likes: 139
G
Britbike forum member
Offline
Britbike forum member
G
Joined: Aug 2001
Posts: 5,741
Likes: 139
Does the bike currently have a no.4 slide fitted? Slide cutaways get leaner with higher numbers.
On this site http://porklips.org/~mason/moto/amal_specs.html
Only the "Sports star" uses a no.4 other ceefers use a 3.5

Last edited by gavin eisler; 04/13/21 2:57 pm.

71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Yes, number 4 fitted Gavin. Comes as standard with the new carb.

AMAL have let me know they are out of stock of the 3.5 slides, but I managed to get a nos one from ebay. Not too many around.

Getting the feeling I've wasted my cash on the new carb! See how this slide helps.....or not.

Joined: Dec 2004
Posts: 11,177
Likes: 114
Britbike forum member
Offline
Britbike forum member
Joined: Dec 2004
Posts: 11,177
Likes: 114
Hmm, slanted is 2 stroke, the 850 Norton one is stepped not slanted. My mistake.

[Linked Image from i.postimg.cc]

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Ok, I'm with you now. Very different indeed!

Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
Hi Doug,
I don’t have access to any C15 manuals, so Gavin’s helpful link is all I have to go on.
There are 9 references to C15’s in the table, only 2 of which include your model year (66), as below:


Type Use Bore Pilot Cutaway Main Needle
Jet Position


375/034 BSA 250cc 1962-66; C15 Star (Home) 7/8" 25 4 140 0.11 4

376/281 BSA 250cc 1962-66; C15 Sports Star (Home) 1" 25 4 200 0.11 2


Only the first uses has a 7/8” bore 375 carb, which I guess is your model, as you have said that scoured inlet is ~7/8” ID.
According to this source, a #4 slide is used.

There are 6 other entries for other earlier models using larger 376 carbs which employ mostly #3.5 slides (one a #3), none a 4. But these are larger bore carbs where a richer slide would be expected due to the lower air speed through the carb.

So “in theory” #4 slide should be correct. However that was then, maybe modern fuel makes a difference?

That the bike arrived with an angled spray tube and the unusual roughing of the inlet port, and that you’ve had this “cough and stall” since you’ve owned it, it seems likely that the PO had made attempts to address the issue, without much success.

As the idle seems to be clean and reliable at ~2.5 turns out, but it makes the issue worse, you are correct to next try a #3.5 slide. No other part of the carb will be involved in the transition from idle.

Folk often set the idling mixture on the rich side (say a quarter turn in) to improve the transition off idle. With your 1.25 turns out setting, you’ve already established that this doesn’t cure the issue, but does alleviate it a little.

If you’re confident of the fuel level in the carb (I’m dubious about the accuracy of the tube method, but many swear by it), and the passages are all clear, then the ~3.5 slide has to be the next step and test.

Best of.

ps. as you have some spares now you could make a float cover with a glass window glued into it, and say if your #3.5 slide improves matters, but not completely, you could use the old #4 slide to make a #3 slide by removing 1/16" from the bottom surface.
This is only to see if the issue is improved by an even smaller cutaway. Then buy the #3 slide, as the modified slide will have changed the needle position in its jet.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Crikey, how did I miss that link in Gavin's post? Must be losing the plot.

Mine is indeed the C15 star that uses the number 4 cutaway. I don't understand the needle jet size of 0.11. As far as I'm concerned the needle jet is a 1055 (or 105.5) as standard. And I've not seen needle position 4 suggested before. Most literature I've seen says position 3. I'll have to check that out more thoroughly.

The bike did arrive with a roughed up inlet, but it has the squared off spray tube. My mention of an angled spray tube was referring to the tube in my 626 carb that Alan was asking about, or at least that's what I thought he was asking.

I like the idea of making a float chamber cover with a window. Will look up a glue that is fuel and ethanol proof.

I'll update here when the new slide arrives.

Joined: May 2013
Posts: 7,926
Likes: 132
Britbike forum member
Offline
Britbike forum member
Joined: May 2013
Posts: 7,926
Likes: 132
A carb gasket will do the job just as good.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
I don’t know why Gavin’s link doesn’t show up as such, try it again.

http://porklips.org/~mason/moto/amal_specs.html

I have no idea about the 0.11 needle jet, I notice its listed for all but a very few applications.
I haven’t had involvement with Monoblocs since about 1977, so will leave that to others.

The needle jet is not connected to your issue, that becomes important at higher throttle openings. The transition from idle merely concerns the idle circuit and the slide cutaway. Don’t get diverted!

Regarding the fuel level window, I think a piece of glass glued with something like Araldite would do, it doesn’t have to survive for years.

So the angled spray tube was in a 626 carb, which has nothing to do with this I guess. Did we get Bantam and C15 conflated?
Anyway, PO thought roughing up the inlet was good for some reason.

I suspect and hope that #3.5 slide will improve matters a lot.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
I think that chart may have the wrong heading. Perhaps 0.11 is the needle itself, and not the needle jet?

I have some araldite or similar, I'll give it a go.

My 626 carb was mentioned earlier as a possible replacement for my old Monobloc, but I went for the new Monobloc.

Yes, fingers crossed for the slide.

Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
0.106" needle jets are the most commonly used by far in both Monobloc and Concentric carbs. I suspect the compiler of that list either didn't know or didn't care? It does raise concerns about the accuracy in general.
Hopefully you have a model-specific Manual? I would trust that.

The needle jet was the only jet to be categorised by ID, all the others are judged by flow. Needles are just part numbers.

As your engine runs well and cleanly at part throttle, the needle/jet setting is probably fine.
This is all about idle/cutaway tiny opening matters.

Don't waste your time on the float bowl window till you've tried the #3.5 slide, unless just curious of course!

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Regarding the float bowl window, I was thinking I could get a piece of clean inner tube, cut it to the size of a float bowl gasket and just press a piece of glass (a strong piece) against it to make a seal. I'd only need to hold it for a while to see what's going on.

Appreciate this is about cutaway right now. I'm not focusing on the needle or needle jet at all.

My old carb had the 106, but should have had the 1055 originally. The new one has the 1055.

Joined: Dec 2004
Posts: 11,177
Likes: 114
Britbike forum member
Offline
Britbike forum member
Joined: Dec 2004
Posts: 11,177
Likes: 114
My C15 had a .1055 needle jet and so does the BSA literature of the day.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
3.5 slide arrived and fitted.

No difference to cough and stall.

Tickover higher until I turned it down.

Tickover more erratic, even when turned down.

I should have pointed this out before, but I keep forgetting to say. Cough and stall is much less when the engine is cold. Only the sharpest of throttle opening causes a cough but no stall. As the engine warms it gets progressively worse.


It almost points to the set up being too rich at low throttle openings rather than weak. Perhaps I should try a smaller pilot or 4.5 slide?

I'm told that the C15 was never supplied with a choke. I'm not sure why this was, but mine does start easily from cold or hot in any weather.

Is it possible for the coil to produce these symptoms at low rpm once it warms up? I get no symptoms once the revs are raised a little. I noticed recently that the clamp has slightly mis-shapen the coil body. Maybe I should try a coil swap with one of the Bantams?

Joined: Sep 2004
Posts: 1,562
Likes: 110
Britbike forum member
Offline
Britbike forum member
Joined: Sep 2004
Posts: 1,562
Likes: 110
I would swap the coil for a known good one, replace the HT lead and plug cap with new ones.

It's also worth checking the rectifier is working and that the alternator is charging the battery. What's the battery voltage like at idle compared to 4k rpm?

Maybe also worth doing a compression check and also check the valve clearances, perhaps the valves arent seating well or rings worn?

Also, check for air leaks around the carb head joint and cylinder head, you may have an air leak somewhere.


1968 A65 Firebird
1967 B44 Shooting Star
1972 Norton Commando
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Will try the coil tomorrow.

Wouid have thought cap and lead would be fine at low rpm as they are fine at higher rpm, although I guess anything is worth a try!

Brand new regulator rectifier is working fine. Producing 6.8v at tickover and 7.2v at anything above tickover. battery drops to 6.2v at rest.

Compression is good and clearances are good. As for seating I've no idea. Also no idea regarding worn rings as yet.

No leaks around the carb or carb/head joint, not really checked around the cylinder head, although there is no visible seepage. I guess I could spray the cylinder head joint in the same way as I spray around the carb joint.

Will update once I've tried everything not covered in your post. Thanks for your input.

Joined: Sep 2004
Posts: 1,562
Likes: 110
Britbike forum member
Offline
Britbike forum member
Joined: Sep 2004
Posts: 1,562
Likes: 110
Interesting that you are using 6v electrics, have you considered changing to 12v, and I wonder which model Boyer are you using, I don't think there are many 6v versions around.

It would be worth checking the Boyer instructions for the EI module you have, it should specify which type of coil is suitable, if it doesn't I would give Boyer a call and ask.

If you have a multimeter you can check the coil by measuring the resistance in the primary and secondary windings and whether there is any leakage between the two or to the body/HT lead terminal.

I don't know what the resistance readings should be on your bike but a general reading should be something like primary winding 3-5ohms and secondary around 10-15k ohm approx.

Last edited by gunner; 04/15/21 7:09 pm.

1968 A65 Firebird
1967 B44 Shooting Star
1972 Norton Commando
Joined: Sep 2013
Posts: 3,061
Likes: 92
Q
Britbike forum member
Offline
Britbike forum member
Q
Joined: Sep 2013
Posts: 3,061
Likes: 92
Originally Posted by gunner
Interesting that you are using 6v electrics, have you considered changing to 12v, and I wonder which model Boyer are you using, I don't think there are many 6v versions around.

It would be worth checking the Boyer instructions for the EI module you have, it should specify which type of coil is suitable, if it doesn't I would give Boyer a call and ask.

If you have a multimeter you can check the coil by measuring the resistance in the primary and secondary windings and whether there is any leakage between the two or to the body/HT lead terminal.

I don't know what the resistance readings should be on your bike but a general reading should be something like primary winding 3-5ohms and secondary around 10-15k ohm approx.

Boyer makes an MKIV ignition ... for 6 volts ... designed to run with an original 6 volt coil
the only caveat is that it needs to be run ...with a 6 volt regulator ...
6 volt dynamos are regulated
6 volt alternators , like a C15 ... didnt come with regulation , the battery was the "regulator"
and "boiling over " helped control over-voltage . ..this is not good enough for a transistorized ignition .

the OP has installed a 6volt regulator ...
and has verified voltages are regulated .
and says he is pleased with ignitions performance ... bike runs smoother through RPMs
but has not solved the stall off idle .
the original coil , if still fitted with the new Boyer , could be part of the original problem ( but I doubt it )

Last edited by quinten; 04/15/21 8:15 pm.
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Boyer obviously sell a 6v EI unit, (as do several others), so I wasn't tempted to change to 12v. All four of my bikes are on 6v and as yet I've had no issues directly related to them being 6v. Even my headlights are excellent. I found a guy selling really good 6v Cree bulbs with bpf fittings.

Electrics isn't my favourite subject, so all the talk about measuring resistance is something I'd have to learn. I've heard that coils can change when they get warm if faulty, but that's as far as my knowledge goes.

In the Boyer instructions it says to remove tank and/or seat to access coils, no mention that a classic bike's coils won't be suitable. It mentions some stuff regarding low compression engines and racing engines, but that's about all.

For now I think I'll simply switch the wires over to a known good coil from one of my Bantams and take it from there.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Thanks for the clarification Quinten. I hadn't seen your post before answering gunner.

I tend to agree the coil won't be the problem, but as it's easy to switch it I'll give it a go.

As for how the bike runs now, I would describe it as "transformed" with the EI and new carb. I was recently hankering to change it for a B40 to get that extra couple of bhp, but I now know I was only running on half the horses available from a C15. It'll probably match my B175s in a drag race now........as long as it doesn't stall!

Joined: Dec 2018
Posts: 189
Likes: 7
D
Britbike forum member
Offline
Britbike forum member
D
Joined: Dec 2018
Posts: 189
Likes: 7
Going back to the carb to head joint ....... I was going to suggest / ask if you had already fitted a tuffnol or similar heat insulation spacer. Is the carb just getting too hot? .......... or studs expanding giving a leak? it is always a fine balance between distorting the carb body and having a inlet leak.

Then I noticed you live in Felixstowe, last time I lived there Polar bears on the beach were a serious problem, I didn't know petrol could freeze. An overheating carb may be less of an issue, but might be worth thinking about.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Hi Dave. I do have a tufnol (or similar) spacer at the inlet, but it is only about 2mm thick, so not sure how much heat that will hold back. My method for sealing them is to use three bond gasket paste on the head side, and then to use a carb o ring on the carb side. This has served me well on all my bikes so far. I have sprayed carb cleaner in this area when cold and warmed up, but the engine never responds to this. If I spray just a tad of carb cleaner into the air filter box, the engine will instantly slow or stop, so I know it does have an effect when it gets in.

As for polar bears and petrol freezing in Felixstowe, I'm not sure if you're taking the pee out of our lovely British weather or if there's another Felixstowe elsewhere!

Joined: Aug 2001
Posts: 5,741
Likes: 139
G
Britbike forum member
Offline
Britbike forum member
G
Joined: Aug 2001
Posts: 5,741
Likes: 139
How old is the ignition switch?, something somewhere has a dodgy connection, earth, fuse holder ,inspect all crimps and bullets from the battery to the switch then the switch to coils. use a multi meter set to ohms across any suspect joints, particularly across any switches in the chain. Anything that flickers or reads much higher than zero is a potential fail. This is the worst sort of fault to find, somthing that resonates at a low frquency and causes poor or intermittent sparks. Im am on my 3rd ignition switch in 40 years of ownership with my BSA.

one very simple test to prove the ignition switch, by pass it completely with a hot wire from battery negative to the coil. IF the fault persists its not the switch. After that if no connections are bad then its trial by substitution. hopefully its just the coil

Last edited by gavin eisler; 04/15/21 11:30 pm.

71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Joined: Dec 2018
Posts: 189
Likes: 7
D
Britbike forum member
Offline
Britbike forum member
D
Joined: Dec 2018
Posts: 189
Likes: 7
Sounds like you have the carb sealing / insulating sorted. I had similar hot running issues before I fitted the tufnol.

The polar bears were real! I lived on a boat for several years back in the '80s upriver from Felixstowe (ferry). The memories of the winters are one of the reasons I now live in the tropics! Iced tea is now a pleasant cooling drink, not what you had to chip out of the teapot in the morning because it had frozen overnight!

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Gavin. Coincidentally, I did run a test on all my wiring whilst fitting the regulator rectifier and also the EI unit. I think the highest I got was 0.02, but most were dead zero. I will try the hot wire method to make sure.

David. Must have been some cold winters up the River Deben in the 80s. A boat can be a dreadfully cold place to live if not heated properly. Perhaps you should have cuddled up to one of those polar bears for warmth!

Joined: Dec 2004
Posts: 11,177
Likes: 114
Britbike forum member
Offline
Britbike forum member
Joined: Dec 2004
Posts: 11,177
Likes: 114
I normally fit a 6mm insulation spacer using longer studs if needed, easy to check the effect or even if the 2mm one is working by going for a run to get engine to temp and then putting a hand on the carb to see how hot it is.

1 member likes this: Allan G
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Next time she is hot I will feel the carb.

Should get time to do the coil swap this afternoon.

Joined: May 2013
Posts: 7,926
Likes: 132
Britbike forum member
Offline
Britbike forum member
Joined: May 2013
Posts: 7,926
Likes: 132
Originally Posted by kommando
I normally fit a 6mm insulation spacer using longer studs if needed
Same here, 6mm minimum to have a reduced heat transference to the carb.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

1 member likes this: kommando
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Tried a known good coil but no difference.
Tried a new plug cap and lead, again no difference.

I ran the bike for 15 minutes until very well warmed up and the whole carb, even quite close to the head is cool. The bowl itself was just room (shed) temperature.

I'm borrowing a compression tester for the weekend, so I won't be doing anything else until I've tested that.

I may have a go at filing my number 3.5 slide to something richer next week when I'm more in the mood for it. Need a short break from it.

Joined: Sep 2013
Posts: 3,061
Likes: 92
Q
Britbike forum member
Offline
Britbike forum member
Q
Joined: Sep 2013
Posts: 3,061
Likes: 92
didnt see this before .
Quote
 I noticed recently that the clamp has slightly mis-shapen the coil body. Maybe I should try a coil swap with one of the Bantams?

a dented coil can contribute to a misfire under load .
i would replace the coil . the coils on other 6 volts bike should be
Compatible ... but the mounting diameter may not be the same .
they come in 2 common diameters 48mm and 40 mm
sometimes this difference in diameter contributes to coil denting ,
sometimes they just get dropped
the dents ... opened up the possibility that the coil can arc to ground
... maybe not all the time , Maybe only more likely when loaded under acceleration

Joined: Sep 2004
Posts: 1,562
Likes: 110
Britbike forum member
Offline
Britbike forum member
Joined: Sep 2004
Posts: 1,562
Likes: 110
Quote
I may have a go at filing my number 3.5 slide to something richer

I don't know if you meant filing or filling, but if it's filing as in removing metal with a file then this will result in a leaner mixture as more air can enter.

On my B44 I'm running a PWK 28 which is a Keihin PWK clone, I had loads of trouble at the start as it was running rich at low rpm. I filed the slide cutaway a bit and it helped lean out the mix.

Another thing to try is to check the throttle cable routing, maybe there is not enough slack and the slide cant bottom or the cable is lacking lube?


1968 A65 Firebird
1967 B44 Shooting Star
1972 Norton Commando
Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
“I may have a go at filing my number 3.5 slide to something richer next week when I'm more in the mood for it. Need a short break from it.”

If the old #4 slide is not excessively worn (say compared to the new #4), I’d be tempted to experiment with that, rather than the new #3.5.
I say that because you will emulate a #3 slide cutaway by removing 1/16” from the bottom surface of a #4 slide (or 1/32” from a #3.5), which will allow you to see if it improves the “cough and stall”, but it changes the slide for ever.

What I’m alluding to is the needle position in the needle jet. This would have to be raised by the corresponding amount removed from the slide. If you have needle slots available to lift the needle by this amount, such a modified slide could still be used.

I am only suggesting to try this experiment with the old #4 merely to see if it makes any improvement to the issue. If it did improve the issue, only then could I support buying yet another slide (#3).

As it seems to run well in all other circumstances, including under load, I consider it unlikely to be an ignition fault (but I appreciate all avenues must be explored).

It has been a long time since I’ve had a Monobloc carb in front of me. Do they run the same 2 hole system (just before and just after the slide) as the Concentric?
On the Concentric these handle the transition from idle to cutaway.

Best of.

Joined: Sep 2013
Posts: 3,061
Likes: 92
Q
Britbike forum member
Offline
Britbike forum member
Q
Joined: Sep 2013
Posts: 3,061
Likes: 92
yes to what gunner said .
file-ing a 3.5 slide will quickly turn it into a 4 ... but probably a slide that makes no sense at all
... you would have to add material ( fill ) to turn a 4 into a 3.5

.. the bottom root angle is started from the same spot on the slide ( that will live next to the idle air Jets )
and the height only increases at the back of the slide .. and is surprisingly small steps
The difference between a 3 1/2 and a 4 ... is 1/32" on the back edge
( the incline angle of the wedge is increasing with slide size ... but measured in 32nds at the back )
from AMAL;
[Linked Image from gallery.myff.org]
a # 2 is richer <<< >>> a #4 is leaner

( more air accross ' the 2 idle Jets' changes how these 2 holes vent fuel or atomized fuel in the idle circuit )

the air screw 'IN' is richer <<< >>> idle air screw 'out' is leaner

there is crossover to what the pilot air screw can do ... with what a change in slide cutaway does .
Opening the idle Jet screw mimics what a larger slide does .

If the change you looking for can't be effected in the the normal operating range of the idle air screw
then the slide is changed ( and idle air returned to default start position )
if neither of these get to where you're going
The problem is probably somewhere else ?
... like between carburetor and inlet valve .(port size)

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Gunner. Yes, filing. My number 3.5 slide has more cutaway than my number 4 slide, indicating that the more you file off the smaller the number gets.

I've copied this from earlier in this thread:
"Slide cutaways get leaner with higher numbers."

So my number 3.5 has more cutaway than the number 4 slide and theoretically creates a richer mixture. So if I file off some more making it a number 3 slide, surely that is even richer?

Some clarification here would be great.

Koan. The slides are both brand new. The number 4 slide came with the brand new carb and is one of the fancy (expensive) hard anodised type, so I'd be reluctant to chop that one about.

From what I've read recently, nothing is taken off the length of the slide when filing. Just the required amount off at the inner end of the slope, graduating down to zero at the outer end of the slope. So the length of the slide is unaffected, as would be the needle position. As far as I can tell, my number 3.5 and number 4 slides are exactly the same length.

Yes, the two tiny holes in the throat of the carb are the same as the Concentrics.

Compression tested tonight. 120psi. That sounds ok to me, but I need to check what it is supposed to be when I get a chance.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Quinten: I'm going to have to compare my two slides again. I was pretty convinced my number 3.5 has more cutaway than my number 4. Maybe I got that back to front? I will check in the morning.

Joined: Sep 2004
Posts: 1,562
Likes: 110
Britbike forum member
Offline
Britbike forum member
Joined: Sep 2004
Posts: 1,562
Likes: 110
Theres some detail about slide cutaways for Concentric AMAL's on their website Here, near the end of the article which is about rebuilding the carb.

I know you are using a Monobloc but I believe the principle is the same, i.e. the larger the cutaway number the leaner the mixture.

120psi cylinder pressure sounds good, see this article from the BSA OC website where someone has measured their C15 - Link


1968 A65 Firebird
1967 B44 Shooting Star
1972 Norton Commando
Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
Not unsurprisingly, after all this, you have become confused.

If a slide had no cutaway at all, it would be a “0”, and it would exert maximum draw on the fuel in the venturi (ie as rich as it can get).

From there the cutaway goes in increments of 1/32” between increments (say between 3 and 3.5), getting progressively leaner as the numbers rise.

Whichever slide you choose to modify, in order to emulate a #3, you will be removing material from the BASE of the slide (1/16” from a #4, 1/32” from a #3.5).

I appreciate what Quinten has posted about the different angles of cutaway in original slides, but that shouldn’t matter much with your rough-and-ready filing experiment to see if a still richer slide helps the issue.
As I see it, it is only a test to see whether buying yet another expensive slide (a #3) is a worthwhile gamble.

Hopefully you are clear that I’m talking about removing material from the BOTTOM of the slide, not the cutaway.
A lathe would be ideal, but careful marking and filing would do it for test purposes.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Apologies to all, I definitely got myself confused for a while.

I was clearly wrong about my 2 slides, I was just the only one that didn't realise it!

The 4 slide does indeed have a greater cutaway than the 3.5, as shown in the picture. Number 4 slide being on the right.

[img]www.dropbox.com/s/hxs1jnzd0jm1xev/Screenshot_20210416-225722_Gallery.jpg?dl=0[/img]

I'll give it some thought over the weekend as to what course of action to take next.

After reading the article in gunner's link I can see 120psi is very good. I will run a compression test on my Bantams tomorrow as a test for the test gauge, just in case it's reading high.

Last edited by Douglas Clark; 04/16/21 10:25 pm. Reason: Link not working
Joined: Aug 2001
Posts: 5,741
Likes: 139
G
Britbike forum member
Offline
Britbike forum member
G
Joined: Aug 2001
Posts: 5,741
Likes: 139
Originally Posted by Douglas Clark
Gunner. Yes, filing. My number 3.5 slide has more cutaway than my number 4 slide, indicating that the more you file off the smaller the number gets.

I've copied this from earlier in this thread:
"Slide cutaways get leaner with higher numbers.", A, a number 4 slide is leaner than a number three, a number 4 has more cutaway to let more AIR in.

So my number 3.5 has more cutaway than the number 4 slide and theoretically creates a richer mixture. So if I file off some more making it a number 3 slide, surely that is even richer? .
A. No, Your number 3.5 slide has Less cutaway than your number 4.

Some clarification here would be great.
A. see above.

Koan. The slides are both brand new. The number 4 slide came with the brand new carb and is one of the fancy (expensive) hard anodised type, so I'd be reluctant to chop that one about.

From what I've read recently, nothing is taken off the length of the slide when filing. Just the required amount off at the inner end of the slope, graduating down to zero at the outer end of the slope. So the length of the slide is unaffected, as would be the needle position. As far as I can tell, my number 3.5 and number 4 slides are exactly the same length.

A, you can only file a slide leaner,BUT, you cant file it richer, taking metal from the bottom will make the cutaway richer but it will drop the needle at the same time cancelling some stuff out.


Compression tested tonight. 120psi. That sounds ok to me, but I need to check what it is supposed to be when I get a chance.


if it is a slide cutaway thing, you cant file up a number 3, what happens if you choke the carb intake, try a hand over half to start with, and then open the throttle, if choking with your hand helps then get a no.3.

Quite often if the port has been opened out in the past normal carb settings dont apply, a hogged out port has low speed air at idle compared to std , lower air speed over the jets gives a leaner mixture, AFAICT

Last edited by gavin eisler; 04/16/21 10:39 pm.

71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
I try the choking idea tomorrow Gavin. Nice idea.

Just another thought. My air filter is the original type in the battery box area. By the looks of it, it offers very little in the way of filtering. I don't think it would offer any resistance to air flow either. A filter working properly is going to restrict air flow to some extent, acting a bit like a choke. Could this perhaps be part of the problem?

Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
If I’ve understood correctly, the only problem you have is coming up from idle (maybe 1000 rpm and zero throttle) to perhaps 1/16 throttle?

It idles well, and runs fine on the road in normal conditions? So just that transition from idle to running?

I don’t see that pointing to an air filter problem (it should starve at higher demand), try it without for thoroughness.

I don’t see it pointing to an ignition problem (it would have to be very specific in rpm).

The compression seems reasonable. I don’t see beyond carburation.

That the PO did that strange roughing of the inlet is a mystery, but I don’t think that would cause this problem, but who knows?
Many thousands of C15’s must have been happily enjoyed over the years, has anyone heard this as a common complaint?
I’ve heard generally a little dull, but dead solid little bikes. So what is different about your bike?
Nothing that I can see, but for that scoured inlet.

Best of.

koan58 #846324 04/16/21 11:30 pm
Joined: Sep 2013
Posts: 3,061
Likes: 92
Q
Britbike forum member
Offline
Britbike forum member
Q
Joined: Sep 2013
Posts: 3,061
Likes: 92
its easy to get backwards or sideways .
i may gets it sideways myself tomorrow .

the slide # is
based on the height of the 'air-wedge' at the air cleaner side
Measured in 1/32 inch increments .
if the cutaway were modeled as a triangle
point A remains fixed ... near the idle jets ...
only line B-C needs to change ... to describe a different slide #
( this is my interpretation of AMAL speak )

( angle-A and line A-B do not need to be found or described ,
they change , but are functions of the height over the
fixed-radius of the slide )
[Linked Image from study.com]
to measure ,
the slide can be placed on a flat surface
and the height of the air-gap at the back measured
( this may seem counterintuitive , measuring the missing part )
but this allows
The change in air volume for various slides
to be gauged simply by the height of the high-side
[Linked Image from mathworld.wolfram.com]

Last edited by quinten; 04/17/21 1:36 am.
Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
I think you're right Quinten, except that point A is about the mid-point of the slide for the smaller cutaways, and further back for the 2 larger ones. (according to your own pic).
The cutaway is always restricted to the inlet side of the slide (of course), so it has to be a slice through the cylinder between that height (say 3/16" for a #3) and the centre, at maximum.
Its only the smaller inside dimensions of the tapered surface of the slide cutaway that matters, the taper may help with gas flow though.
Most applications use the 3 middle options.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Koan. Your understanding of the situation is spot on. The bike could be ridden as it is, as long as I'm prepared to have a high tickover or keep the revs higher with the throttle at traffic lights etc. The higher rpm alleviated the cough and stall somewhat.

I have read in the past things like "the dreaded cough and stall" or "the curse of cough and stall", both relating directly to the C15, which lead me to believe this was a common problem. However, I can no longer find those comments on the interweb.

Regarding being "generally dull', I'll bet a lot of that comes from AAUs not working properly after not much use. Also by 1966, faster more exciting Japanese bikes (especially 250s) were starting to arrive making the C15 look like an old plodder. The EI has given me back a proper reliable (hopefully) advance curve, and a much more pleasing bike to ride.

Another thought regarding that rough inlet. Is it possible that the roughness is trapping fuel on tickover, and then releasing some of it (or all of it) as the throttle is opened causing greater suction? This would cause a momentary over rich situation which could cause stalling once the engine is warm. Clutching at straws here!

Quinten. Thanks for the further clarification. I think I have the basics of slide cutaways now. I'm a slow learner, but hopefully I'll retain it now that I know it! The main thing I will concentrate on is: higher number = more cutaway = more air.

Joined: Dec 2004
Posts: 11,177
Likes: 114
Britbike forum member
Offline
Britbike forum member
Joined: Dec 2004
Posts: 11,177
Likes: 114
Have you tried the Colortune yet, opening the throttle at tick over and observing the colour of the combustion will tell if its a lean or rich issue or even if the problem is the lack of spark.

1 member likes this: Allan G
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
That's a good point Kommando. The guy I borrowed the compression tester from also has the colourtune. I will try and get my hands on it later today.

Joined: Dec 2018
Posts: 189
Likes: 7
D
Britbike forum member
Offline
Britbike forum member
D
Joined: Dec 2018
Posts: 189
Likes: 7
I am sure you will have already done this but ........
Before you start hacking bit off the slide I would just one more time go through this procedure

http://amalcarb.co.uk/downloadfiles/AMAL/amal_tuning_guide.pdf

don't rush it and don't try and skip steps! .

It strikes me that the issue is the transition between the pilot and the cut away, so either, or both, could be the cause (on indeed something else entirely!).
The cut away only comes into play over about 1/8 throttle. So if it "fluffs" at less than this it isn't the cut away, its the pilot and visa versa over 1/8th. Marking the twist grip (with tape) so you can actually see at what throttle position the "fluff" or "cough and stall" actually occurs might be useful. In my experience even a 1/8 turn on the pilot can make a big effect.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Thanks for the link to the tuning guide Dave. I'll put that on my list of things to try. I'm not planning to attack the slides at this point. They're expensive and there's no going back, so I'll save them for another day.

I am about to go and get a colourtune kit from my friend. It will be interesting to see what that throws up.

Gavin mentioned choking it using my hand over the back of the carb. I'm hoping to take that a step further. The C15 never had a choke (air slide) fitted. If my Concentric air slide fits in the Monobloc slide, I am going to fit that. I have a lever and cable I can use. That'll make it much easier to experiment with choking.

Busy afternoon ahead!

Joined: Sep 2004
Posts: 1,562
Likes: 110
Britbike forum member
Offline
Britbike forum member
Joined: Sep 2004
Posts: 1,562
Likes: 110
My first ever bike (apart from a Yamaha FS1E DX) was a BSA C15 SS80 I rebuilt from parts and a wreck when I was 18.

It was a great bike and I learnt a lot, it pains me now to think that I sold it all those years ago for £40, wish I could have it back.

I wish you luck with the colourtune and hope it solves the problem.

If the colourtune doesn't help then I would start checking things like valve timing. It wasn't unusual for BSA to miss stamp some of the timing wheels which means your timing may out one tooth advanced or retarded. This is mentioned in the rupert ratio unit singles Manual so could be a possibility.


1968 A65 Firebird
1967 B44 Shooting Star
1972 Norton Commando
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
If you'd kept the FS1E DX (maybe you have) you'd be able to sell it and comfortably buy 2 X SS80s. Crazy world!

I will update here re colourtune, the tuning guide and choking the carb a little.

Then as you say, I might have to start looking deeper into the mechanicals. Valve timing, sticky valve etc.

I'm still interested in my theory that fuel may build up in the roughness of the inlet port, it really is very rough. To sand it all smooth would add a good 1/16" to the diameter. Another way would be to fill it with something that flows well and sand back to 7/8". A tricky job for DIY, but there maybe a professional out there that would do it?

Joined: Aug 2001
Posts: 5,741
Likes: 139
G
Britbike forum member
Offline
Britbike forum member
G
Joined: Aug 2001
Posts: 5,741
Likes: 139
There is a port smoothing goo out there, normal epoxies get dissolved by fuel, I have some of the special stuff, had to import it from the states.
I will dig it out and post a link later. its a 2 part thing, applies easily and sands easily, turns black with the fuel eventually but crucially doesnt dissolve.
This is a more involved solution but may work a lot better, primitive tuners/ port attackers often remove too much material particularly where the port turns down at the approach to the lowest part of the valve,this should be a large radius which has no sharp turn at the valve seat, if its been removed/ attacked , you will see big improvements in flow and carburettion if it is rebuilt.


71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Joined: Aug 2001
Posts: 5,741
Likes: 139
G
Britbike forum member
Offline
Britbike forum member
G
Joined: Aug 2001
Posts: 5,741
Likes: 139
The stuff you want for porting is " Kneadatite A/B Epoxy putty." from PSI Polymeric Sytems , inc.
The data sheet has an EU contact.
Whitford Plastics Ltd
10, Christelton Court
Manor park
Runcorn
Cheshire
WA7 1ST
tel, (44) (1928) 571000
email, [email protected]
website: whitfordww.com

i used this to reshape the butchered ports in my A65, its surprisingly easy to use , make sure the ports are fully degreased with an aggresive solvent before applying. most /all of it went into filling the port floor and rebuilding the bottom turn to the valves. Obvs, the head and valves will need to be removed for this work, . Allow 24 hours to cure then sand / file to shape.
[Linked Image from i.postimg.cc]

Last edited by gavin eisler; 04/17/21 4:43 pm.

71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Wow. Seems there is a product for everything out there. Thanks for that. I will email or call on Monday to find out where suppliers are should I need some.

I've just had 10 minutes playing with the colourtune kit.

Tickover is good, nice and blue.

All rpm above cough and stall are blue or orange depending on throttle position. Settles on blue when the rpm is steady, whatever the rpm is.

During the cough and stall area, which I now know is probably between 1/16 and 1/8 throttle it is very yellow. As I very gradually apply the throttle from tickover, there is initially a very bright yellow, and if I then hold the throttle it steadies to a not quite so bright yellow, but definitely yellow all the same. I did try winding the mixture screw out, and it did make a slight difference, but after 3.5 turns (one turn out from standard) it didn't seem to make any difference.

If I blip the throttle there are two scenarios. 1) she coughs and the colourtune goes yellow then instantly black and she dies. 2) she coughs, there is a huge yellow flash but this time she survives and after that split second the revs build and blue running is resumed.

So this may have proved quite quickly that I'm not looking at too weak, I'm probably looking at too rich.

A couple of days back I started suspecting this, so I ordered a number 4.5 slide and a number 20 pilot. Hopefully they will be here early next week for further testing.

I remember further back in this thread I said things seemed slightly better when I screwed the mixture screw in, but I'm now wondering if I did this before sufficiently warming the engine. In other words, it was happy with a bit more fuel when cold.

Progress perhaps?

Last edited by Douglas Clark; 04/17/21 9:32 pm. Reason: Spelling.
Joined: May 2013
Posts: 7,926
Likes: 132
Britbike forum member
Offline
Britbike forum member
Joined: May 2013
Posts: 7,926
Likes: 132
Originally Posted by gunner
. It wasn't unusual for BSA to miss stamp some of the timing wheels which means your timing may out one tooth advanced or retarded. This is mentioned in the rupert ratio unit singles manual so could be a possibility.


A lot of these parts got thrown back into a different parts bin during its time on the assembly line then 50-60 years later get sold on eBay as NOS.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Joined: Dec 2018
Posts: 189
Likes: 7
D
Britbike forum member
Offline
Britbike forum member
D
Joined: Dec 2018
Posts: 189
Likes: 7
it is many summers since I last used colourtune, and then on a car, so I must admit I watched a Youtube vid, and it confirmed my memory.
The "huge yellow flash" is to be expected, or even desired, as the mixture is momentarily rich as the throttle opens, it needs to be.
What I find interesting (from what you wrote) is that when it isn't "huge" the motor dies on transition between pilot and slide.
What is also interesting is that this doesn't happen every time.

Now this might be barking up the wrong tree in entirely the wrong forest but ...... all machines are made by human hands, there isn't a bit of swarf rattling around under the pilot jet is there?? sometimes restricting sometimes not ?

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Hi Dave.
To be perfectly frank I've not studied the pilot jet in much detail, so I've no idea if there is something in there restricting flow. I assume though it would cause a week mixture if there was a restriction, and I'm inclined to think now that it is over richness causing the issue.

Just to repeat something I said earlier:
"As I very gradually apply the throttle from tickover, there is initially a very bright yellow, and if I then hold the throttle it steadies to a not quite so bright yellow, but definitely yellow all the same."
I believe under normal circumstances when rolling the throttle on very gradually, I shouldn't be getting that bright yellow followed by a steady yellow when the throttle is being held still. I think what you are describing is a more normal throttle opening speed where you would indeed expect an initial richening, but my concern is that it remains yellow on a steady throttle at that particular point, but nowhere else in the rev range.

Also, it has always been hit or miss as to whether she coughs and stalls or coughs and recovers, even with the old carb. I think some of it depends on the speed the throttle is opened. The faster (and wider) it is opened the more likely it will stall.

I will however take a close look at the pilot jet in case there is something going on in there. I'm not intending to try much else before trying the new weaker slide and/or the weaker pilot jet. Hopefully both will arrive within a couple of days.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
An update.

Dave: no swarf visible in or around the pilot.

I have now tried a number 5 slide and a 20 pilot, both weaker than standard. Despite my confidence, I appear to be totally wrong. It was noticeably worse using these two items. I tried them together and individually. With the smaller pilot it was harder to start, and with the slide I was getting popping or backfire from the exhaust on the over run. And plenty of cough and stall.

So I went back to standard pilot (25) and the 3.5 slide which is richer than standard number 4. This time, I started with the mixture screw just 1 turn out. With this richer slide and mixture setting there is no detectable cough and stall, but, the idle is all over the place. I gradually weakened the mixture adjusting for tickover as I went along, and found a reasonably steady (far from perfect) tickover at 2 turns out, but at that point the cough and stall had returned.

So all those that said cough and stall was from being too weak do appear to be right. So maybe a 30 pilot, or number 3 slide? Maybe a better air filter? I'm sure mine does nothing to restrict air flow like a normal filter would. I'm at a bit of a loss!

Last edited by Douglas Clark; 04/24/21 5:04 pm. Reason: Missing words
Joined: Sep 2004
Posts: 1,562
Likes: 110
Britbike forum member
Offline
Britbike forum member
Joined: Sep 2004
Posts: 1,562
Likes: 110
There's a bit of info on the Trials Central website Here (third post down), where someone mentions they have all but cured the cough and stall on their C15 trials bike.

Apparently the fix involved adding 2 thick tufnol spacers on the manifold and also using a 35 pilot jet. The poster mentions the airway behind the pilot/air screw has been drilled out in order to allow a bigger pilot but doesn't know how it was done.

Don't know if this will fix your problem but I thought it was worth sharing.

Last edited by gunner; 04/24/21 5:51 pm.

1968 A65 Firebird
1967 B44 Shooting Star
1972 Norton Commando
Joined: Dec 2002
Posts: 692
Likes: 8
M
Britbike forum member
Online Content
Britbike forum member
M
Joined: Dec 2002
Posts: 692
Likes: 8
Originally Posted by Douglas Clark
So I went back to standard pilot (25) and the 3.5 slide which is richer than standard number 4. This time, I started with the mixture screw just 1 turn out. With this richer slide and mixture setting there is no detectable cough and stall, but, the idle is all over the place. I gradually weakened the mixture adjusting for tickover as I went along, and found a reasonably steady (far from perfect) tickover at 2 turns out, but at that point the cough and stall had returned.

Pardon me if you're already answered this but are you testing and adjusting with the engine at normal operating temps? Most engines like a richer mixture when cold but this should improve as the temps come up. I'm assuming that this behavior continues to occur even at operating temps but correct me if that is incorrect.

One simple test for me would be to take the air cleaner off and, using your left hand, place the tip of your finger in the airstream, blocking part of the cutaway. With your right hand, give it throttle and see if there is any improvement. This will mimic a different slide cutaway and help you narrow down whether a different slide would help.

If it does, doubly ensure that there isn't an air leak at the mounting surface before changing anything. You want to be certain that you're not leaning out the mixture elsewhere. If it all checks out, start with a richer pilot jet and that's likely cheaper than another slide.

I hope that helps.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Thanks for that Gunner.

I don't know how he got 40mm worth of spacers in. Couldn't do that with mine, not enough room! I'm guessing as it's a rltrials forum his bodywork is gone. What would the spacers do? An area I have zero knowledge of.

I was also interested in the guy further down that said the conical filter he bought made his run too weak. If that's the case, my virtually straight through air filter won't be helping. I think I need to get this area sorted before playing around with jets and slides any more. I'll look into getting a decent filter.

MarcB #847032 04/24/21 6:34 pm
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Hi Marc.

I turned the air screw in to 1 turn out before starting the bike, but I then fully warmed her up before trying to make further adjustment. I've learned the hard way that warming up is essential.

I tried blocking part of the air flow before, but I think I was not being careful enough to not overdo it as the engine would cut out. I will try it again being a bit more careful. I think I was blocking too much.

The mounting surface (to the barrel) is the one area I'm very confident I've got sealed. No amount of squrting carb cleaner in that area affects the running. Although sometimes it can be hard to tell when you have a tickover that's up and down like a yo-yo.

As I mentioned earlier, I think my air filter is doing very little and probably letting too much air through. My friend has a screw on type of filter. I might see if that will squeeze in the gap to see what difference that makes. That's assuming the thread size is correct.

Joined: Sep 2004
Posts: 1,562
Likes: 110
Britbike forum member
Offline
Britbike forum member
Joined: Sep 2004
Posts: 1,562
Likes: 110
Regarding the air filter, I would avoid fitting a foam one as pictured in the link I posted earlier.

I once used a similar looking filter on my B44 and one day when restarting the engine, it kicked back and spat back through the carb. To my horror I saw flames appear and the filter was on fire. Luckily with gloved hands I was frantically able to extinguish the flames before the whole bike went up.

After that experience I've never used foam filters and rely on the K&N style with metal gauze or original pancake type which I believe are designed not to catch on fire.


1968 A65 Firebird
1967 B44 Shooting Star
1972 Norton Commando
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Wow, that must have been a scary moment. Thanks for the heads up.

I was indeed looking for a similar foam filter, but luckily I found nothing that would fit in the minimal space between the carb and air filter/battery box, so I didn't get one.

Instead, I have decided to upgrade the original air filter. I have bought a section of foam manufactured and sold by "Ram-Air" filters. Ram-Air have been supplying filters for motorcycles and cars for many years, so I'm guessing this foam is both fuel and fire resistant. It is a rectangular section that I'm planning to cut down into a circle to fit the original air filter cage thingy.

Perhaps I should try and find out a bit more about the safety features of Ram-Air foam before proceeding?

Joined: Dec 2004
Posts: 11,177
Likes: 114
Britbike forum member
Offline
Britbike forum member
Joined: Dec 2004
Posts: 11,177
Likes: 114
I use Ramair foam in my Commando as the OEM foam filter disintegrated, its oiled as the oil tank breather opens out above the filter and copes well.

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
That's reassuring thanks Kommando.

I won't oil mine initially as it might restrict breathing too much. I can always add oil later!

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
https://forum.retro-rides.org/thread/187887/friendly-warning-foam-filters

Maybe I shouldn't have read the above?

Starting to look at ways to avoid foam use.

Joined: Dec 2004
Posts: 11,177
Likes: 114
Britbike forum member
Offline
Britbike forum member
Joined: Dec 2004
Posts: 11,177
Likes: 114
Prior to foam, surgical gauze and then paper was used as the filter material, just as inflammable. Then you have the use of carb cleaner as a starting fluid.

Quote
The quickest way to get it going is to spray a small amount of carb cleaner into the body... just a squit... and it will fire up off that and then pull in fuel thereafter. Not ideal but it's what I'm working with at the moment.

1 member likes this: Allan G
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
I guess using starter fluid on a vehicle prone to carb backfiring was a tad risky!

I've since read about some fires caused by using oil soaked filters (spray oil) before the propellant and/or solvents in the oil had been allowed to dry out.

My filter foam doesn't arrive for a few days, so plenty of time to consider my options.

I've also ordered a 30 pilot jet, so I'll give that a whizz when it gets here to see what difference it makes.

Last edited by Douglas Clark; 04/25/21 2:20 pm. Reason: Punctuation
Joined: Sep 2004
Posts: 1,562
Likes: 110
Britbike forum member
Offline
Britbike forum member
Joined: Sep 2004
Posts: 1,562
Likes: 110
I think that if you are using the original air filter housing with foam as Kommando has done then the risk is minimal since the foam is enclosed.

The problem that I had was that I wasn't using a housing and the filter was simply a foam body with a mounting to bolt onto the carb, similar to the one in the link. I think the filter became quite flammable after a while as it probably soaked up oil from leaks and etc. and without a metal body there was nothing to help contain the flames.

So I think you would be OK using foam as long as it enclosed in the original housing.


1968 A65 Firebird
1967 B44 Shooting Star
1972 Norton Commando
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Several of the fires I've read about have been with filters directly connected to the carb and also not enclosed. A few of the riders were able to grab the filters with gloves hands and tear them off to save the bike.

What I have in mind is using the usual 2' rubber tube from carb to air box and placing the foam sheet inside the original filter housing. So maybe that'll be safe enough. I'll give it some more thought over the next few days.

Who'd have thought that someone that has been riding for 50 years with undoubtedly all sorts of unseen dangers, would now be weighing up the safety of something that is generally considered safe and normal?!!

Joined: Dec 2004
Posts: 11,177
Likes: 114
Britbike forum member
Offline
Britbike forum member
Joined: Dec 2004
Posts: 11,177
Likes: 114
Just wait until you get a leaking carb and a kickback lights the petrol, does not matter what the air filter element is made from or even if one is fitted.

Joined: Nov 2012
Posts: 2,067
Likes: 59
K
Britbike forum member
Offline
Britbike forum member
K
Joined: Nov 2012
Posts: 2,067
Likes: 59
Perhaps I’m barking up the wrong tree, but its unusual to want more intake resistance to air flow.

Such increased flow resistance will have little impact at low flow (ie low rpm/low throttle) but will have richening impact at higher rpm/high throttle.

As you’ve got the foam on its way, I guess you’re gonna have to try it to satisfy yourself. I hope it has the desired result, and I will be eating my thoughts!

As the engine runs well in all other respects, there aren’t many other things to look at.
The scoured inlet is the only one I can think of.
The choke cable entry has been sealed?

From what we’ve heard so far, I’m unsure whether “cough and stall” was a usual problem with C15’s or not. (I’ve not had one).

If it was a usual problem when newish, then perhaps it’s a design problem with the inlet port?

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
"its unusual to want more intake resistance to air flow."

I suppose I'm working on the basis that the engine was originally set up with an air filter that did actually provide some resistance, whether required or not. It was just an unavoidable consequence of wanting to filter the incoming air. So all I'm trying to do is achieve something similar to the original set up. My current air filter might as well not be there. It's just metal gauze with a very thin and moth eaten piece of cloth that only covers a small area.

When I've had running issues in the past with other bikes, one of the questions that crops up regularly is regarding air leaks in the air intake. Well I couldn't have a much bigger leak than my current air filter!

"As you’ve got the foam on its way, I guess you’re gonna have to try it to satisfy yourself"
If nothing else it will provide some basic filtration. I've really no idea if it will go any way towards fixing my problems. As you said, only experimenting will tell me that for sure. I take your point that it might cause a greater effect at higher rpm and could be detrimental.

The choke cable inlet is sealed from new on my old carb and new one. I'm told the C15 never had a choke fitted.

I'm keeping the scoured inlet in mind if all else fails. I have some Kneadatite in my sights just in case.

"I’m unsure whether “cough and stall” was a usual problem with C15’s or not."
I've definitely heard comments in the past about "the dreaded cough and stall" and there is some talk about it in the trials scene where a lot of these bikes have ended up. As someone mentioned before in this thread, maybe the scouring was a previous owner's attempt at curing it?

I've got the foam on order and also a 30 pilot. I'll see which one arrives first and have a play with it.

Joined: May 2013
Posts: 7,926
Likes: 132
Britbike forum member
Offline
Britbike forum member
Joined: May 2013
Posts: 7,926
Likes: 132
Originally Posted by gunner
I think that if you are using the original air filter housing with foam as Kommando has done then the risk is minimal since the foam is enclosed.

Eh?

If the filter can be soaked with fuel and can be exposed to oxygen then it will ignite, regardless of what it is encased with. After having knowing someone who’s rocket 3 set on fire in this same way I think that is proof enough.

The foam filters should be covered/impregnated with filter oil. This makes them work efficiently and are ideal for dusty environments. They work better also when impregnated with oil than when dry.

Paper filters work well but will eventually get wet by fuel or water and work less efficiently.

The K&N filters are good also, TB-100 is the one that fits in the old
Pancake filter housing, folk often over oil them and wonder why performance sucks, they are also quite the premium these days. Any modern cotton filter will work just as well in a typical ideal environment (ie not dusty)


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
I just had a look at K&N TB-0100 and it's not the filter that would fit my machine. Does it perhaps go in the screw on pancake type filter?

Joined: May 2013
Posts: 7,926
Likes: 132
Britbike forum member
Offline
Britbike forum member
Joined: May 2013
Posts: 7,926
Likes: 132
Hi Douglas, apologies for any confusion.

The Tb-0100 would replace the paper filter elements that come in the pancakes with the threaded piece and top and bottom plate. Such like fitted on monoblocks and Concentric carbs.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
Ok, I know the ones you mean. If there was room I would have fitted one of those to mine. Unfortunately there is less than an inch between my carb and the airbox🙁

Joined: Aug 2015
Posts: 1,755
Likes: 37
A
Britbike forum member
Offline
Britbike forum member
A
Joined: Aug 2015
Posts: 1,755
Likes: 37
Originally Posted by kommando
Just wait until you get a leaking carb and a kickback lights the petrol, does not matter what the air filter element is made from or even if one is fitted.

I'm surprised that more bikes don't catch fire. A start procedure that involves making the carb overflow and leak petrol onto a source of ignition, the magneto is far from ideal.
BSA must have had a bike catch fire as they decided to fit a drip deflector on the A7 and A10


BSA B31 500 "Stargazer"
Greeves 200 "Blue Meanie"
Greeves 350
Greeves 360
Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500 sprint/LSR bike "Deofol"
Jawa 500 "Llareggub"
Aprilia RSV Mille "Lo Stregone"
'35 & '36 OK Supreme
Kawasaki ZZR1400 "Kuro no senshi"
Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
And how many riders back in the day had a lit ciggy on when tickling those carbs?

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
As Koan said, the air filter did badly affect higher rpm and also to some extent the mid range. Seeing as the carb is all standard, I can only assume BSA fitted a very free flowing filter as standard. I have removed the filter and she now revs cleanly right up to high rpm.

The 30 pilot jet had a similar affect to the air filter. I know the carb tuning charts show that the pilot jet has zero effect on wide open throttle, but I have to disagree strongly with that. I have messed around with pilot jets on several Bantams and they always have an effect on every part of the rpm range. The 30 pilot today had quite a significant effect at wide open throttle. It would not rev out, (felt like a rev limiter) and actually started to bog down. I replaced the pilot with the standard 25 and now she revs cleanly again.

With the air filter and/or the 30 pilot fitted the cough and stall was improved a little, but as I've said above, the rest of the bike was ruined.

So it looks as though whatever I do to alleviate the cough and stall, problems turn up elsewhere. I'm sure in the hands of a better carb tuner than me, this could be sorted, but for now I'm just going to live with it.

I've been talking to a guy that knows a good bit about tuning racing Bantams and C15s for trials, and he has advised that I should get that scoured inlet port sorted
(back to standard) before trying anything else. So I'm going to use the bike for this season as it is and get the head and barrel off in the winter to try and sort it. I will revive this thread if I ever make any progress, but for now I'm afraid I'm done. My brain can't take anymore.

Thank-you to all that have taken the time to help. Much appreciated.

Joined: Aug 2001
Posts: 8,622
Likes: 68
T
Britbike forum member
Offline
Britbike forum member
T
Joined: Aug 2001
Posts: 8,622
Likes: 68
Long thread, didn’t read, but if we’re talking about those BSA/Triumph pancake air filters with paper elements, I gave up in them a long time ago. Obviously restrictive and prone to cracking in the front plate, through vibration.


Amateur Loctite enthusiast.
Joined: Sep 2013
Posts: 3,061
Likes: 92
Q
Britbike forum member
Offline
Britbike forum member
Q
Joined: Sep 2013
Posts: 3,061
Likes: 92
[Linked Image from ttr250.com]
[Linked Image from ttr250.com]

Joined: May 2020
Posts: 100
Likes: 1
D
Britbike forum member
OP Offline
Britbike forum member
D
Joined: May 2020
Posts: 100
Likes: 1
That's exactly what I have Quinten.

Mine is 3 metal plates and one piece of very thin cloth.

I separated the parts and just used the thick top plate with a circle of Ram-Air filter underneath. Turned out to be way too restrictive.

Page 1 of 6 1 2 3 4 5 6

Moderated by  Allan G, Jon W. Whitley 

Link Copied to Clipboard
Job CycleBritish Cycle SupplyMorries PlaceKlempf British PartsPodtronicVintage MagazineBSA Unit SinglesBritBike SponsorBritish Tools & FastenersBritBike SponsorBritBike Sponsor






© 1996-2021 britbike.com
Powered by UBB.threads™ PHP Forum Software 7.7.5