I have a B&S 15,5 OHV engine on a Craftsman mower. Saturday it mowed fine for an hour and then suddenly it started missing and backfiring through the muffler.

I replaced the fuel filter, pulled the carb and blew out all the passges with compressed air, confirmed the needle and seat work, everything was very clean. I pulled the valve cover and confirmed the valves clearences. I replaced the spark plug. All to no avail. The engine starts well at idle and you can get the RPMs up if you gently increase throttle but there's no power. If you accelerate quickly or put a load on the engine (engage blades or transmisson for example) it backfires through the muffler repeatedly, misses and dies. Pulling the spark plug at that point reveals a totally saturated wet plug.

HELP! What should I be looking at?

Mark

I'm thinking a sticky valve and most likely the exhaust valve that hangs open. Pull the muffler and spray the heck out of the exhaust valve while turning it over by hand some. May take a few treatments. Maybe spray through spark plug hole towards the valves while turning it by hand or cranking the starter. Maybe run some marvel mystery oil in the gas and oil.

The wet plug tells me the intake valve is working fine. After the valves, I'd think about the coil but you can check to see if your are getting spark when it starts acting up. Backfiring is also indicative of a sheared key in the flywheel throwing the engine out of time but I don't suppose it would run good at all if that was the case.

Keep us posted with what you find out.