Hi! I'm new here, hope y'all can help. I was given a Troy-Bilt 42 inch Tuffy. 18.5hp B&S OHV single cylinder. Getting spark (grounded plug against engine,) and getting fuel (plug was wet from trying to start.) I've used it once since it was given to me. Took awhile to get running due to old gas, but have run all of that plus a half tank new gas. Stalled yesterday. Spins strong. Started to adjust valves and found that bottom (I think exhaust?) valve barely opens. Rocker arm hardly moves to compress spring. Went ahead with adjustment but still won't start. Any ideas or advice? Thanks!!