Originally Posted by Baloo
Hello. My 258 just started backfiring out of the exhaust a couple days ago. I havent driven it since then. When I crank it, it even backfires when idleing. I've tried adjusting the idle and mixture screws on the carb, but that doesn't work. I just don't have a clue. I'm learning as I go here and I haven't come across this yet.
Possibly a piece of carbon between exhaust valve seat and face. This would keep valve from being able to shut completely. You can get a fuel additive that will clean fuel system and valves. Don't know if it will work but it won't hurt anything.
To eliminate the carb as the problem, slowly close choke (by hand)while idleing and see if it stops or gets worse. If you are getting a backfire due to a lean condition of the carb, this should temporarily richen it up and cause it to stop.