i've got a bobcat walk behind with the fh451v, after letting it warm up i went to throttle it up, the motor back fired and changed pitch. now it has no power as in it barely pulls the blades. it starts and runs but is super rich, and fouls plugs. i've been all though the carb, changed the gas out, it seems to be getting gas, and has new coils. i've even pulled the muffler off and checked to make sure each pulley turns freely. it almost seems to be out of time, but i don't know how that could have happened instantly. i'm at a real loss, any ideas? thanks john.