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

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.

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.