Was mowing when engine stopped running as if I had turned it off. Found an exhaust rod broken. Replaced rod, replaced the fly wheel key since I already had it apart. Cleaned the gas tank, fuel line, carburetor and air filter also. When I start it tries to run but backfires and will not run correctly. I had replaced the spark plug, no help so put the old one back in just as a test. Took the OHC apart and cleaned the valves. Still have the same problem with back firing. Remember that this is a riding mower so no bent shaft. Any suggestions?