Info on Four-Stroking
"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 in cases where it doesn't occur normally even 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 wide-open throttle without load [ie: going downhill or reaching top RPM without upshifting]. In the latter case this happens because the air-fuel mixture is 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 wide-open throttle without load is not normally done in most applications.
Retarding ignition timing reduces four-stroking, as it allows more time for scavenging to take effect. Two-stroke engines, when running at low power, are less sensitive to ignition timing changes than four-stroke engine. If the timing is retarded for low throttle positions, from perhaps 35° BTDC at normal speeds to TDC or even 10° after TDC at slow idle, the engine runs well without four-stroking.
When a four-stroking engine eventually fires, the excess mixture from the previous stroke causes an excessive cylinder pressure. This can be nearly double the normal pressure, leading to excess noise and potential failure of overloaded bearings in the connecting rod.
Four-stroking is particularly noisy, especially as it occurs when the engine is otherwise relatively quiet and the vehicle may be stationary in neutral gear.
Irving, P.E. (1986) . Tuning for Speed (6th ed.). Menoshire. ISBN 0-908031-29-7.
— (1967). Two-Stroke Power Units. London: Newnes.
— (1976). Rich Mixture: A Motorcycle Miscellany. Australia,: Research Publications.
— (2004) . Motorcycle Technicalities. Turton & Armstrong Pty. ISBN 0-908031-82-3.
Although the writer of the Wikipedia article denounces a too rich mixture as causing four-stroking [maybe he was only thinking of normal no-load four-stroking] he lists having a restrictive air filter as contributing to this (which causes a richer mixture and limits flow which restricts the loop flow in the cylinder to expel the exhaust gases). I would add that engines like a rich mixture under load and like a lean mixture without load. So a mixture set to handle maximum load will always be too rich under a no-load situation.
This quote is from a chainsaw forum: "The proper tune for a chainsaw is to run it rich enough to four-stroke at wide open throttle with NO LOAD, but to smooth out the instant load is applied."
Since a one speed motorized bike can't keep upshifting to keep the engine under load we then experience a "light-load" situation at wide-open-throttle (WOT) at top speed which is similar to the no-load situation the author writes about. Leaning the carb mixture to avoid this means the carb will be a tad too lean while climbing hills or just when accelerating up to top speed.
My Suzuki 100 had a horrible rat-a-tat-tat combustion/exhaust sound at low throttle just poking along. I replaced the stock CDI and coil (2-in-1) with my Jaguar CDI and a high voltage coil of the right mounting hole distance (nothing special but a decent coil) and after setting it up the irregular combustion problem went away. I know the low RPM timing is more retarded with my CDI. So this backs up what the writer of the quoted article said about retarded timing helping to alleviate the problem. I know the Jaguar CDI is more retarded at all different RPM than the standard Grubee CDI. So if you can't get rid of four-stroking by carb tuning then you might want to try the Jaguar CDI.
Also my motored bike (55cc with 18mm Mikuni) is jetted right on and I get no four stroking. I didn't have to lean the mix to prevent it. I attribute that to the Jaguar CDI. The stock CDI just has way too much spark advance.