Small Engines (Lawn Mowers, etc.): Tecumseh OHH60 Stalls Under Load, darlington pair, fuel starvation

Question
Eric
I have a tecumseh 6.5 HP ohv go cart engine that runs good for a few minutes until it heats up and then the timing seems to mess up? The carb inlet vacuums the intake air properly and then it starts to blow out the carb instead of keeping the vacuum when the engine is running at high speed? I thought the valves were bad so I installed a brand new head/pcv valve and adjusted the valves to the proper clearances. I ruled out the carb because there is no way for the fuel to get into the combustion chamber when the pressure blows backwards out of the carb intake. This causes a fuel starvation. If you let off of the throttle the engine will correct and stay running until you open up the throttle and run it for another 20-30 seconds. The fuel breather is not plugged. I do know that the rings are worn as the engine smokes a little. Could this cause this condition if the valves are working as they should? I am lost? Could the compression release cause this under load? It only does it when the engine is hot and running fast. Thanks

Answer
Have you checked the flywheel key?  I have seen partially sheared flywheel keys.

Do you have a spark igntion coil you can try?  Some coils have an ignition advance, internal to the coil, usually a darlington pair transitor set up, that could be malfunctioning when hot.  This is very rare but I had one coil that had this problem on a chainsaw.

What are the valve clearances?

Eric