6 volt flywheels have 139 teeth. 12 volt flywheels have 168 teeth. Be sure you have the correct flywheel/starter combination. A 6V starter has a 5/8" diameter armature shaft, and a 12V starter shaft is 1/2" diameter. 139 tooth flywheel- - - -5/8" starter shaft. 168 tooth flywheel- - - -1/2" starter shaft. Both starter gears have the same number of teeth, but the pitch is different. It sounds like you have a starter/flywheel mismatch.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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