My riding mower performs beautifully but after about 40 minutes the engine surges very fast, then slows considerably. This process repeats endlessly. After the engine has cooled, it runs perfectly then eventually starts surging again.
I put in new plugs, new gas filter, changed the oil. Help!
Robert::(

I have a 21HP Craftsman, 5 lawnmowers, 6 weedeaters and they all do this if the Air Filter is dirty. Just a thot, maybe you should clean or change air filter.