I have a mower with a 14 hp Briggs and Stratton motor that runs great until I put it under torque, then it back fires and wants to die. I have replaced the spark plug and cleaned the carburetor. What is my problem with this motor? Timing? Valves not adjusted?

Hi,
It could be that the valves could use an adjustment, but it could also be that the flywheel key is partially sheared. One thing that happened with newer engines and their associated ignition coils is that they would run, though not as well as normal with the key sheared or partially sheared. This is the result of building in a spark advance system that allowed the engine to be EPA emissions compliant.
Peace,
Clarke

The flywheel key is OK and I have adjusted the clearance for the intake valve to .004 and the exhaust to .006 It seems to idle better at lower rpms but still backfires when under torque.
Are the clearances correct? The model number is 28p777 Type 0644-A1

Hi,
Believe I would try a new plug gapped at ).030".
Peace,
Clarke