Author Topic: C15 Cough and Stall  (Read 1168 times)

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DAVE BRADY

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Re: C15 Cough and Stall
« Reply #15 on: 12 June, 2021, 09:13:16 »
Hi Gary,

I have had two B40s in the past and both ticked over OK and I know of several C15s that covered many reliable miles and ticked over OK.
Some uniform roughness is better than a polished surface as it helps to cause turbulence and results in a more thorough mixing of the fuel and air.  Yours may be a bit too course but I doubt it would have too much affect on slow running unless the inlet has been opened up.  Is it the correct size?  Has the head been tuned/gas flowed for a bigger inlet valve and hotter cam?  If so then the standard carb settings may not work too well.

Dave.
 

Gary

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Re: C15 Cough and Stall
« Reply #16 on: 12 June, 2021, 09:40:49 »
Hi Dave.

The tickover is a bit random, but it's the cough and stall on throttle application that is the bigger problem, although I suspect they are related.

You're not the first to say the inlet being rough should be ok, and others on a different forum also agree it may be a bit too coarse, but shouldn't be problematic.

I had a theory that on tickover, fuel could gather in the grooves in the inlet, (they are very coarse grooves) then as they overflowed an extra bit of fuel would get taken into the combustion chamber altering the tickover. Once they filled again they would overflow and do the same again, dozens of times per minute perhaps, hence a random tickover. On application of the throttle the extra air rushing through would empty the remaining fuel in these grooves and cause a temporary flooding, hence the cough and stall.

This theory had a less than luke warm reception on the Britbike forum, but I'm still quite keen on it🙂
Apparently there is a special type of filler used in modelling that could be used to fill the grooves, but I felt reluctant to trust the filler not to fall out at some point, so I forgot that idea. Not sure what tool could be used to return it to standard, except perhaps a long finger and some Emery cloth.

The inlet is 22mm, same as the carb, so that seems ok.

I have absolutely no idea regarding the state of tune of the head/valves/cam etc. Maybe I'll find out one day if I decide to strip it and take those grooves out.


DAVE BRADY

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Re: C15 Cough and Stall
« Reply #17 on: 12 June, 2021, 10:10:07 »
Hi Gary,

Usually the cough and stall from tick over is related to pilot jet and slide cut away and as far as I know it is a weakening of the mixture in that transition/overlap to needle jet as the slide is raised that causes it.
I would have thought that with the 'fill and over flow' theory it would smooth out once the engine was running even at tick over.  If you have a choke fitted does it still have the same problem with the choke lowered.  This effectively reduces the slide cut away so richening the mixture.
Another bit of kit that I find useful is a 'Colortune'.  If you are not familiar with it , it replaces the plug with one that has a glass bit.  You can see the colour of the ignition and the affects of altering the pilot screw.  When you open the throttle you can also see if the mixture temporarily becomes richer and settle to the correct blue colour so checking the needle/needle jet and the main jet.

Dave.

Gary

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Re: C15 Cough and Stall
« Reply #18 on: 12 June, 2021, 11:44:25 »
I have tried a number of different pilot jets , both larger and smaller. All change the characteristics but not the cough and stall. I've also tried larger and smaller cutaways in combination with the different pilot jets, but again, they all caused changes, but none were desirable!

I have used colourtune in combination with the various pilots and cutaways, and it all looks to be normal except for when it stalls, and the glass then shows nothing. There was always a richening (orange flame) just before it stalled.

All the experiments are on a normal opening of the throttle. If I snap it wide open it stalls instantly every time. Only by feathering can the stall be avoided completely. During normal use its 50/50.

There is no choke fitted as standard on these bikes, and as such, the new carb also came without one. I bought a handlebar lever, cable and slide as I thought the choke would help with experimentation, but when the slide arrived it was the wrong one. At the time there were none available that I could find, but maybe that has changed now.


B Murphy

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Re: C15 Cough and Stall
« Reply #19 on: 12 June, 2021, 14:17:31 »
Hi Gary

Try a OKO carb and see if that makes a difference, they are only approx 24.

Gary

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Re: C15 Cough and Stall
« Reply #20 on: 12 June, 2021, 14:31:31 »
I did look for an oko carb a few months back as someone else suggested it, but I couldn't find one to fit. Either the inlet was too big/small or the stud holes were wrong. Maybe I was looking in the wrong places?

When I get time I will check the valve clearance. If ok, I'll take the head off and see what I find in there. Just a bit reluctant to go for another carb now, especially as I've already spent well over 200 on the new Amal.

I'm getting the general message from the forums that these bikes can run fine as long as everything is ok. It would seem this isn't the widespread problem I thought it was. so I will persevere for now until I find my issue or until I've had enough of it!

courtney

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Re: C15 Cough and Stall
« Reply #21 on: 13 June, 2021, 12:38:47 »
Is it worth trying a lead additive as others have mentioned fuel could be the issue? Might this make any difference? I'm just thinking out loud, I have no idea.
1965 BSA C15
1970 BSA B25 (US spec.)
2004 Aprilia Tuono

Gary

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Re: C15 Cough and Stall
« Reply #22 on: 13 June, 2021, 14:19:44 »
Thanks for your suggestion.

I use redex lead replacement from time to time (when I remember) but I don't remember a time when the cough and stall disappeared. I haven't used it lately, so I will add some for the next ride.