The engine is going as fast as it can go. The mower just isn't moving fast. When I first start it, it runs fine for the first 10 or 15 minutes and then it slows down to a crawl, especially going up any incline. My yard is not steep. It may have a 5% incline at most. Could it be the drive belt slipping? And if so, is there a "tension" adjustment on the pulley?
Drive belt tensioners are usually self adjusting, courtesy of a strong spring that pulls the belt tight. The first thing you need to do is to have a look at the drive belt to see if its split or apparently worn - or maybe covered in oil. See if any springs are actually connected at both ends, too - they are known to snap.
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