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The aim of the ignition point is to get the maximum pressure from the burning fuel/air charge to occur at top dead centre in order to apply maximum thrust on the piston. The speed at which the fuel burns can be considered to be constant, (it’s all a lot more complicated than that but good enough for now). However as engine revolutions increase the time for the crank to rotate by one degree decreases therefore to give the mixture enough time to achieve maximum pressure the spark has to occur earlier and earlier as engine revolutions increase.

So the “Book of the Scott” is correct and the scooter forum is wrong.

If you wanted to get really technical the book “Fuel Economy of the Gasoline Engine” (ISBN 0 333 22022 6) by D.R. Blackmore & A. Thomas is a good read. But if you get a migraine trying to understand it all don’t blame me!