Chainsaw Repair

From the Wikipedia article:

Two stroke engines rely on effective scavenging in order to operate correctly. This clears out the combustion exhaust gases from the previous cycle and allows refilling with a clean mix of air and fuel. If scavenging falters, the mixture of unburnable exhaust gas with the new mixture may produce an overall charge that fails to ignite correctly. Only when this charge is further diluted, by pumping through a second volume of clean mixture, does it become flammable again. The engine thus begins to 'fire-and-miss' every second cycle (every four strokes), rather than correctly on every cycle.  Four-stroking begins gradually, so the engine first starts to run with an unpredictable mixture of two- and four-stroke cycles. When severe, this may even become six- or eight-stroking.

"If scavenging falters" is not an explanation or statement of cause.  You're running a chainsaw at maybe 10,000rpm, and something associated with a modest increase in rpm causes a missed ignition - the mixture fails to burn.  The first question is why?  There is no explanation here. 

The next revolution takes 6ms, and the rpm cannot appreciably change in that time.   The throttle is still wide open, and the air coming into the cylinder passed through the same carb.  Regardless of what caused the misfire, what has changed now that it will fire this time?  This was the main question I was trying to get to - I'm already pretty convinced of what causes it to begin with, although I know others don't share that belief.  To my mind it would seem that the lack of a firing means there are different pressures in the cylinder, and pushing back down the transfers, and that somehow this changes the mixture and conditions in the cylinder making it more conducive for firing next time.

Scavenging of small two-stroke engines relies on inertial scavenging through the Kadenacy effect. At low rpm and low gasflow velocities, this effect is reduced. Scavenging thus becomes less effective when idling, and so it is when idling (at either low rpm or low throttle) that four-stroking is most likely to become a problem. Schnuerle or loop scavenging is considered to be less prone than the simpler cross-scavenging.

And yet chainsaws are loop scavenged, and the effect we're discussing happens at high rpm.

Four-stroking is not caused by an over-rich mixture, as is widely believed, although this can make it worse.[note 1]

This is simply an assertion without reference or support.  Note 1 merely discusses block air filters. 

Unfortunately there is little substance to this article and I didn't learn anything from it. 

thought it was lack of air?

Hey Joe!  Lack of air into the cylinder?  What would change about air flow between consecutive firings at 10,000rpm?