Riding mower keeps back firing and will not start. briggs and stratton 21.0 hp 540cc

Backfiring will not be caused by the governor. Generally backfiring when trying to start is caused by the timing being out, to check this you would need to remove the blower housing and the flywheel, check to see if the key in the crank has sheared. If it has go to a Briggs & Stratton dealer with the serial numbers that are on the blower housing and get a new key, job done if the key is OK then check the carb for flooding, check to see if the oil level in the engine has risen a lot, if it has then good chance that the carb has flooded on fuel has gone into the sump and is burning under the piston causing a backfire or if not that then check valve clearances, as a guide 0.06" on the inlet 0.08" on exhaust ( this is only an approximate)

Need to reset the govanor.