Sheared Flywheel Key
If you have a lawnmower that is backfiring or pulling the pull start out of your hand, the flywheel key may be sheared. The key in a small engine mates the flywheel to the crankshaft in a certain spot. The flywheel can only fit one way or the timing will be incorrect. The flywheel key can be sheared if a heavy object was hit by the mower, such as a rock. With no key keeping the engine in the right place it will no longer run correctly.
