Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board Cough and stall

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!