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Does the coil just connect to a set of cam-operated mechanical points, or does it use a magnetic pickup or other electronic means? It may help to use a transistor to drive the coil and then create the spark precisely when the points open, without arcing, which can result in reduced power, lower voltage spark, and delay of sufficient spark to ignite the mixture. That may be why some engines have to have like 30+ degrees of spark advance, even at low speed. If you are interested I may be able to help, Here is a circuit diagram and simulation: