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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.

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.?

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.

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

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.

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).

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.

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.

Thanks Gavin. I'll give that a try.

Lots to do tomorrow!

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.

Copied from a thread on the triumph board, I think John described your issue perfectly....

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.

Douglas, the figure is ".0035" clearance" which is 3.5 thou.

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).

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.

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?

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!!

I would guess a Concentric 926. Just a guess, folk here will have more experience.

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.

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?

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.

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.

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.

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.

Is the spray tube angled or cut square?

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.

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.

Will do Allan, thanks👍

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

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?

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.

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.

I interpreted that photo as showing the inlet port, if so it is the ideal surface finish

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👍

“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.

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.

"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!

Try a slide with the next size smaller cutaway, if it has a 3 .5 fitted , try a 3.

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.

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.

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.

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.

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?

I'd like a £ for every time I've heard that........and another £ for every time it turns out to be true!!😂

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.

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.

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.

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.

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

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.

Hmm, slanted is 2 stroke, the 850 Norton one is stepped not slanted. My mistake.

Ok, I'm with you now. Very different indeed!

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.

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.

A carb gasket will do the job just as good.

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.

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.

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!

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.

My C15 had a .1055 needle jet and so does the BSA literature of the day.

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?

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.

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.

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 )

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.

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!

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.

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!

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

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!

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!

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.

Next time she is hot I will feel the carb.

Should get time to do the coil swap this afternoon.

Same here, 6mm minimum to have a reduced heat transference to the carb.

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.