The starter spins but does not engage the flywheel to start the engine. No teeth are missing from the starter gear or flywheel, the pinion just does not move up to engage the flywheel. This has happened intermittently in the past, but up to now would eventually engage and start. Tried using jumper cables on the battery terminal to be sure there is enough current, but no difference.

That part of the starter with the gear is called the Bendix. The torque of the starters spin should cause the gear to slip down the candy cane groooves of the bendix. Take it apart, clean it and lube. Or buy a new bendix.

Hi,
Battery is hooked up backwards.
Peace,
Clarke