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Join Date: Jan 2011
Location: Smack in the middle of Illinois
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Sorry I missed this the day it was posted - the most common problem when it does what you describe is the starter drive on the flywheel is defective - the starter drive is an over sized Bendix or one way clutch. On a rare occasion, the teeth on the starter motor shaft simply wear off. There may be other causes, like a loose flywheel and a sheared flywheel key or the bolts securing the starter drive to the flywheel may have come out or (and I have seen this happen) the battery is installed reverse polarity and the starter is spinning backwards. Finally, I have seen starters internally wired incorrectly or wired for reverse rotation - if the starter was replaced and then the problem reared it's ugly head, it might be a reverse rotation starter - if the starter has not been replaced immediately prior to the problem, then rule out the motor's direction rotation due to design. It's relatively easy to pull the starter motor to inspect the teeth cut into the armature. If the teeth are good, then go for the starter drive.
The starter drive is easiest accessed by removing the engine - if removing the engine is not desired, it can be accessed by removing the swing arm.