classic motorcycle forum

I have a Vincent Comet. It will tick over hot or cold and at any time at traffic lights or junctions etc. Normally no problem but sometimes it will spit back once through the carburettor and stop. Restarts immedaitely and will then tick over. Could be ok for the rest of the day.
Why does it spit back? I have checked:
Ignition timing
Valve timing
Good compression
Tappet clearance
Spark plug (tried several alternatives) new lead. Recon mag, points
Mixture
jet sizes
Alternative carb

I'm baffled!