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#843806 03/22/21 10:26 pm
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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.

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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.


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

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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.

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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

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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.

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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).

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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.

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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.


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Thanks Gavin. I'll give that a try.

Lots to do tomorrow!

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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.

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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.


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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.

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

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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).

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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.

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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?

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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!!

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

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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.

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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?

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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.

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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.

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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.


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

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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.

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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)

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