Four-stroking

Four-stroking is a condition of two-stroke engines where combustion occurs every four strokes or more, rather than every two. Though normal in some instances at idle, extremely high engine speeds, and when letting off the throttle, such firing is uneven, noisy and may, in cases of malfunction, damage the engine if allowed to continue unabated.

Four stroking will occur in a correctly adjusted two stroke engine at full throttle without load when the air-fuel mixture becomes overly rich and prevents the engine from running faster. At such high speeds too lean mixture will cause the engine to over-rev as well as overheat, and in engines running on premixed fuel a too lean mixture will cause poor lubrication.

In chain saw operation, where natural fluctuation of chain bite during a cut can cause momentary over-revving, the full throttle mixture is adjusted for four-stroking to occur at a set high rpm, cutting engine speed and enriching lubrication.

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 enriched by a second volume of clean mixture does it become flammable again. The engine thus begins to fire 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.

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.

Four-stroking is not caused by an over-rich mixture, as is widely believed, although this can make it worse. Nor is it caused by excessive oil/fuel lubrication mixtures.

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Wikipedia