Hey guys new here but in desperate need of help I am a starting small engine repair man and have been fighting this lawn mower for two weeks now. It tries to crank but after a few rotations it kicks back like it hit something. I have changed the flywheel key three times, torqued it and readjusted the valves too many times to count. I have taken the carb apart cleaned and nothing. I have a BS 15.5 31A607 0458 E1. Please help somebody.