This mower was running fine , I drove into the garage after mowing a medium lawn , shut it off and it was never to run again. Crankshaft would only rock a little with channel locks and yes it has oil in it before and after. Replaced ten horse with a thirteen horse brigs sinngle out of a Toro Wheelhorse 1338, mine was a 1038 and it ran for about a hour and then backfired , then would not restart. Let it cool overnight and then it started this morning . What would cause the backfire and then no restart ! Help?