I recently inherited a Sears Eager 5HP walk-behind (self-propelled) from a neighbor. It sat for some time unused and when I got it there was no spark plug installed. After installing a new plug and changing the oil I found it ran poorly so performed the usual -- Overhauled the carb and inspected the flywheel key. The engine now starts and runs excellently...for 5 minutes. As soon as the engine comes too full operating temperature it loses what seems to be about 20%-25% of its power. The RPMs are definitely dialed down (probably by 600-700) and the walk-behind speed is noticeably slower. It still mows fine but not with the speed or gusto it should. 2-cycle I'd be thinking piston/sleeve wear, what might be the likely culprit with this 4-stroke?