Possibly the firing order got messed up.
Valve timing off, timing chain jump.
Your car backfires when you need a new transmission,or tune up. How do you know the difference? Simple...if your car backfires then wont start, or wont start and then backfires,you need a transmission. If it backfires, and then starts moments after, you need a tune-up. Glad to help...David
Your timing belt has probably worn out, or broken, and has jumped timing.
It could be your carborator. Check that out. If your using Holley get rid of it. That is complete junk.
spark plug gap, gas/air mixture on carb.
could be lots of things is engine light on ? is there a vacuum leak check for a crack in the intake tube from the air box to the throttle body if engine lights on get code
check timing belt/chain sounds like valves not timed properly, or not closing completely
Sounds like the timing is off.
sheared flywheel key. youtube has vids... type in mower cord backfires & you'll see instructions on how to fix it.
The engine is out of time. Have the engine timing checked. It can also be the plug wires are connected incorrectly.
check fuil mixture