While this has not happened on my Kohler, I would replace the spark plug (ensure its gapped to specs). In most cases of backfiring, its usually sometime to do with the ignition system, followed by fuel, followed by carburetion. Usually fuel-related backfiring is when the engine is cold, so that does not match with your problem. However, a partially clogged fuel filter could be the issue. If carburetion, check automatic choke-related so WD-40 the choke assembly and ensure it moves freely and open and closes fully. Spray carburetor cleaner into intake while running to clean out ports. Good luck.
