I have a Toro 6.5hp 22" rear drive walking behind lawn mower. Today I began mowing the lawn - started normal - and after 4 passes it sounded as if the engine was under load and then stopped running. When I attempted to restart the mower I pulled the rope and there was no resistance and I could hear the flywhel spinning. Did I shear the flywheel key or is there something else I need to look for? Please advise.