Small Engines (Lawn Mowers, etc.): kohler cv-22 backfire, latter problem, scag

Question
I have a cv22 on a scag ssz. It starts fine and runs OK for about 20 minutes. At 3500 rpm under a load it loses power and backfires through the exhaust after 20 minutes. After the problem starts I can return to idle and everything is normal after a few minutes. I have replaced both coils, advance module, plugs, fuel pump and fuel filter.

Answer
This one has me a little stumped. I'm leaning on taking the carburetor off and cleaning it thouroughly. Aside from that, you could have a valve problem or an engine wear problem. Depending on the age of your product, it may well be the latter problem.