Small Engine Troubleshooting Problems? Ask a Mechanic Now.
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Have you checked the intake gaskets where the intake mounts to the cylinder block? This engine has a history of the intake gaskets leaking after several hours of use. If you want to check this, start the engine and spray some carburetor cleaner around the base of the intake on both sides where the intake mounts to the cylinder and see if you notice any difference in running.
In that case I have no other suggestions for you. I will opt out of you question, maybe one of the other experts here will pick up on it and have another suggestion.
Is the exhaust port closed with carbon buildup?