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Weak spark can result in low performance and lower power at high speeds. Possible causes for weak spark are going to be the coils spaced incorrectly from the flywheel or there is a concern where as the stop diode is sending some ground to your coils. I would disconnect the stop diode and see if you get the same results. If you do not then check and see if the diode is getting a ground signal. You can stall the engine if it does run well by choking out the air intake.
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