Usually, When a lawn mower engine has extremely hard/strong compression, there are three things to check. First, remove the spark plug and turn the engine over. Make sure the valves move freely, and there is no real resistance when you turn it over. (I think you already did this.) Second, leave the spark plug wire off, and try to turn it over. If the compression feels normal, then the issue is probably Your flywheel key. If not, remove the head and gasket, and clean all the carbon off of the head, piston, and valves. Also, make sure the rings aren't leaking oil into the combustion area. When you reassemble the head on the engine make sure you use the right gasket. If the above doesn't work, I've had success by removing some material from the head itself. Use a dremel tool and carefully remove some aluminum from the very edge of the combustion chamber, on the side away from the valves. Anyway, hope this helps, and best of luck!
Zachary Worsech
March 2020
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