Small Engines (Lawn Mowers, etc.): Mower stalls, wont start after it get shot, briggs stratton, hp model

Question
I have a Briggs & Stratton 3.5 hp (model 92502) engine on my snapper mower (circa 1979).  It starts and runs fine but after using it for about 20 minutes it stalls out and wouldn't start.  I recently replaced the oil, sparkplug and cleaned the air filter.

Answer
It sounds like a couple of things could be happening. This could be caused by the fuel tank vent being blocked. Try loosening the cap if the vent and running. If it corrects the problem, replace the cap. Another possibility could be an ignition problem caused by heat. You can connect an inductive timing light on the plug lead(s) and see if the ignition cuts out when the engine dies. Another issue could be incorrect valve lash. You would need to adjust the valve lash to the correct clearance. Hope this helps.