I have a Craftsman #917.378941, OHV-7-HP self propelled lawnmower that I need help diagnosing an engine problem. It sounds like its knocking, maybe two knocks, when I first try to start it. It runs for a short period, and then the blade loosens up, it starts to knock again, and I then torque it back down and it runs for another short 30-second period. Then the blade loosens again. When the blade is loose, I can see that the crank shaft turns 180 degrees, but the fan blades donÂt, then it catches and the fan turns. The engine, when manually rotated, spins without any noise or great effort. I pulled on the starter a few times and it eventfully pulled the starter handle out of my hand. Could this be a "sheer-pin" somewhere in another part of the engine? ItÂs not an older mower, maybe 5-years old at the most. Thanks for your thoughts and help in advance!

John