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Dirt Bike Mods & Maintenance
Ignition timing on 2 stroke
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[QUOTE="Sherby24501, post: 946014, member: 57484"] Timing effect on a gasoline engine are usually such that at lower volumetric efficencys (lower load) the engine requires more advance to give the best torque. Unfortunately at low speeds detonation (pinking) is caused by end gases starting to burn before the flame front from the spark plug propagates to them (usually only below 6000 revs/min). The noise is caused by the flame fronts coliding (like thunder). Over advance (past minimum advance best torque) puts a higher load on the piston (may caus more mechanical noise) and cools the exhaust gas (more complete combustion before exhaust event) this changes the effect of the expansion chamber because the speed of sound in the exhaust gas is linked to temperature. Excessive over advance also puts an extra load on the plug (heat has to go somewere) and may cause pre-ignition - this is what typically causes holes in pistons, but the leaning of the mixture too far also increases the burn rate (and hence reduces the advance required) and increases the exhaust gas temperature. The effects you descibe could be due to an overly rich mixture increasing the ignition requirements and the pipe tuning better due to the change in exhaust gas temperature. It is worth noting that the stroke of the engine effects the real timing when using a Dial gauge, but your sons is actually more advanced than yours ! maybe yuors was retarded as originally set (which may explain the very poor performance). There is a lot of mis-information on the web and in general I would suggest that you try to establish the ignition timing so as to be safe (overadvance is bad for bottom ends). and concentrate on getting the fueling right. Also try using a colder plug if messing around with the ignition as these on 4 strokes (especially high output) are often the weakes link. Hope this helps [/QUOTE]
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MX, SX & Off-Road Discussions
Dirt Bike Mods & Maintenance
Ignition timing on 2 stroke
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