micka, if you have checked the camshaft timing, I think we'd all like to know what you found.
There are only three things that seem able to cause steady puffing back through the carburetor: wrong ignition timing, wrong camshaft timing, or leaky inlet valve. However when I re-read your description, you may have meant that occasionally, at unpredictable intervals, it pops back through the carburetor, and if the air cleaner is removed, it may be a sort of ball of flame that is expelled out into the air. This is quite different from what I thought you meant up to now.
Those occasional fireballs from the carburetor are called "spits" or "lean spits", and are a symptom of lean mixture. They mainly happen when you have just applied load (i.e., just after the throttle has been opened by the governor).
If you have spits rather than steady puffing, there is an intake leak or a fairly severe carburetor problem, calling for a strip and clean, with new gaskets where necessary. The intake leak is the more likely on an old engine that may have had amateur maintenance along the way.
Last edited by grumpy; 13/05/11 03:31 AM. Reason: Add possibility of lean mixture
