Thank you in advance for your help. Removed gas head for new intake valves. Reinstalled head and I am sure I set the intake valve clearance at .017 and exhaust valve clearance at .019. (Mfg. says .016 intake HOT, and .018 exhaust HOT). Switched spark plugs to determine if bad plug. Switched wires to see if bad wire. Tractor still refuses to run on all 4. Input appreciated.