What is "four-stroking"

n. (context automotive English) A type of abnormal operation of a two-stroke internal combustion engine where the cylinder only fires every second cycle (like a four-stroke engine does). This occurs when the exhaust scavenging and inlet charging leaves a mixture in the cylinder which does not ignite, but needs a second scavenge/charge to produce an adequate mix.

Four-stroking is an operating condition of two-stroke engines, where they instead begin to fire every four strokes or more, rather than every two strokes. This firing is uneven, noisy and may even, in cases where it doesn't occur normally, damage the engine if allowed to continue unabated.

However, in some circumstances, four-stroking is normal. When idling most two stroke engines will four-stroke, as well as when letting off the throttle.

Four stroking will also occur in a correctly adjusted two stroke engine at full throttle without load. In the latter case this happens because the air-fuel mixture becomes overly rich and prevents the engine from running faster. The engine is intentionally constructed by the manufacturer for this to happen, as a 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. Running the engine at full throttle without load is not normally done in most applications, but in a chain saw the full throttle mixture is actually adjusted for the engine to four stroke at a given rpm set by the manufacturer. This is done by adjusting the high rpm screw on the carburetor while the engine runs at full throttle until the correct rpm level can be read on a tachometer.