Re: vespa ignition timing

James G /
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To answer the original question in very simplistic terms:

Wider point gap = more advance = better high-end

Your plug fires sometime shortly before TDC. This ignites the mixture, which burns quickly but not instantaneously; it is not a high-explosive. It burns in a short but non-trivial amount of time. As the flame front propagates from the spark out towards the cylinder walls the piston is still moving up towards TDC, then to TDC, then away from TDC. Sometime very shortly after TDC the mixture will be completely burnt, the heat will have expanded the air and force will be applied to the downward stroke that the piston was just starting.

At high RPMs the piston is moving faster linearly than at low RPMs, so if we say that "t" is the time from spark to total burn, the piston will cover more linear distance in the same time "t" at 6000rpm than it would at 2000rpm. Taking this into account, if you are tuning for high-rpm power, you will likely want ignition to occur slightly earlier than you would if you were tuning for low end grunt. Given a very retarded timing at high rpm the piston will have traveled well past TDC and the pressure of the combustion process will reach it later in its down stroke, when compression is consequently lower and thus with reduced power.

On the other hand, given too much advance you could get your power hitting the piston while it's still on the upstroke, which will very rapidly destroy your top end. Even a small amount of advance can make starting a little more difficult because you may then need to get the piston moving faster before it will fire. This can be very noticeable on kick-start motorcycles, where a bike may take a much longer kick or swifter kick to start after advancing the timing.

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