Small Engines (Lawn Mowers, etc.): Engine backfire and shearing flywheel key

Question
I have a Craftsman 46"cut lawn tractor model #917.28852 powered by a 21 horsepower Briggs & Stratton single cylinder OHV engine model #331877, type #2371-G5. It ran and mowed fine the first year and for several months this year. The last time I mowed it was running fine when I parked it in the barn, but when I tried to start it about a week later to mow again the engine wouldn't turn over. I had the battery tested and it was OK. I took it to a repair shop and he found that the valves were out of adjustment and it had sheared the flywheel key. He adjusted valves and replaced flywheel key.  It started and ran fine, but when he turned it off, it backfired and sheared the flywheel key again. He repeated this procedure several times with the same result. What could be causing this?

Answer
Bent push rod(s)?  You can remove each push rod, one at a time and noting which end goes inside the engine, and then roll each one on a piece of glass to see if the rod(s) is bent.

Excessive carbon build up in the valves?

While rare to shear flywheel keys on rider engines, I would use a little lapping compound and lap the flywheel to the crankshaft.

Do you know what the valves are being set to (the gap)?

Eric