was using my machine for about an hour today running just fine, when i got a flat on the right rear tire. I got out to look at the tire with the machine running the engine backfired. I jumped back in and shut it off, trying to figure out what had happened. looked over the engine and at looked to be OK. Then got back in and re-stared the machine and drove 60 to 80 feet to hard ground where I could change the tire. the machine backfired again before i shut it down. what kind of problem could this be? What should I be looking for? Or is it because the engine was running and the machine tilting back from the flat tire. PS: looking for a good was to free the rusty lug nuts on the flat, having one hell of a time with them.