Join Date: Jun 2009
Location: Otero County, NM
First off, it's easy enough to pop off the starter and verify that electrically and mechanically it is OK. One thing to try considering that this seems to be an intermittent problem: IF the starter seems to be working OK (or at times OK) but won't engage the flywheel, when that happens, place the transmission in gear and roll the bike a couple of inches and try again.
It can be that you can break or wear a tooth on the flywheel and, if that broken tooth area is what presents when you engage the starter, the starter gear will find nothing to grab and will just spin or rattle if the gear teeth are only partially chipped or worn. By rolling the bike a bit in gear, you change the position of the flywheel and, perhaps, rotate it to an area where the teeth are OK.
If the starter engages then seems to let go before the engine starts, I would wonder about a starter mechanical or electrical issue - including battery.