Small Engines (Lawn Mowers, etc.): 21hp vanguard engine, inductive timing light, fuel tank

Question
a new 21hp vanguard engine runs great for 3-5 minutes. then stumbles and stops. can be immediately be restarted and the process is repeated.

Answer
It sounds like a couple of things could be happening.  This could be caused by the fuel tank vent being closed or 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.