as a retired certified auto tech, I can give you two possible answers, first if this machine has a timing chain I would think that when it warms up the chain becomes longer thru expansion thus making the valve timing wrong and allowing backfires thru the valves. the other is if it has two gears rather than a chain that when it warms up the valves are not seating to an exact fit allowing the backfiring, my guess is that after 3 to 5 hours of cool down time in winter temps that this machine does not backfire when first started, also possible but not likely is the two gears cam/crank are worn , this of course would be if it does not have a chain. So in order of likely hood, chain, valves , gears
michael mantyla, February 2010
