I have a craftsman 25 hp lawn tractor. Found 2 bent push rods in one of the cylinders. Replaced pushrods, adjusted valves (took a few tries), mowed grass for bout half an hour. Shut off engine for a couple minutes, tried to restart, engine backfired and hard to start. Removed valve cover and found exhaust valve pushrod clear off of rocker arm and just laying there. Adjusting nut is tight. Screw that holds rocker arm in place is tight. What would cause this? Intake push rod is ok. Neither rod is bent.