With modern (cleaner) fuels carbon is unlikely, neither on the piston nor on the valves. Maybe the bike is running on the main jet, rather than the idle jet - that would account for a lean mixture and spitting back. But you write that spitting back appears at regular running at higher revs (like a hiccup), right? Anyway, maybe double check the idle system? Try disconnecting the cables completely at the carb, and leave the choke slide out, carb top off. The slide should drop fully and only be raised by the adjuster.

Thanks Thomas.

The timing was indeed out and rectifying that improved things immeasurably.  I also followed the Amal troubleshooting guide and that helped.  I'm still getting spitting back at times and wonder if I have an air leak somewhere.  I've tightened all the things I can tighten on the carb.  Could carbon build up on a valve lead to spitting back when it doesn't close fully?

Hi Thomas,

• The leaning bike - I always have the same effect with my C11 but never with the Sunbeam. it's the fuel level in the float chamber that changes with the tilt and has an effect on the slow running/air correction jet. A weaker mix is likely to speed up the engine (as it does just as you are about to run out of fuel).
• I also had a back spitting carb. The reason was an imperfect ignition timing.

Cheers, Thomas

It's an Amal 276 and the float's on the left side.  I've just discovered that the throttle adjuster nut on the top of the carb is loose.  Having done it up, the bike seems to start promptly and seems more willing to tick over.  Will see if there's a genuine improvement tomorrow before I claim any kind of victory!

What Model Amal carb do you have Thomas,and which side of the carb is the float chamber on.
John

I've had some problems getting my Model 8 to start and then tick over, but following the Amal iterative adjustment process of adjusting throttle stop and pilot jet has got me to a point where the bike starts reasonably well and will tick over.  I then have two further issues:

1. If I lean the bike over to the left to put it on its side stand the tickover begins to slow and then the bike stalls.  I can't figure out what might cause this.  Could it be the float is set too low and I'm suffering fuel starvation?  Could the float be jamming when at an angle?  Debris in the carb?  Any ideas?

2. When riding along I'm getting quite regular spit back through the carb and I think the engine would stall if the bike's momentum didn't immediately restart it.  Reading the Amal tuning guide, could this be because the main jet is too high or too low?

Any insights welcomed!