[Dimis]

You suggest that this happens in frequently... just a slightly different line of thought if I may... I'm no expert.

Have you noticed any common denominator?

Like after a long spirited run where your engine is running a little hotter than normal?
Any chance your carb is oversupplying fuel ie: high fuel pressure or high float at this point (feeding the engine more fuel) causing the engine to run-on after you kill the electrics?

If so, its possible that this run-on may be preventing your engine from cylinder wash.


In any case, as others have mentioned, Lippy's method is a simple fix.