I have a 12.5 HP I/C murray riding mover. When trying to start, it would turn and crank.
But would not turn over and start. It seems as if engine is turning too freely. Checked the sparkplug, it's OK. It was running this season, so it could not be the fuel line, could it? Any help would be greatly appreciated. Thanks.:confused:

Check the flywheel key.

Yep, the flywheel key could be the cause.

What is a flywheel key? And How do I check it or replace it? Sorry for sounding dum.

I can't find any good refererences, but his is a start.

How is the flywheel and key inspected?

The flywheel usually does not have to be removed to inspect it, but removing a flywheel when the flywheel key has been sheared can be difficult. There is a slot in the flywheel hole and a corresponding slot in the crackshft. There should be a contiguous piece of metal (Key) spanning the slots. If the key is sheared the flywheel will free spin without subsequent turning of the crankshaft or the engine will be out of time becase of the wrong placement.

Flywheel key seems to be OK. Not sheared or cracked. What's next??

How free does the flywheel move?
How free does the shaft move?
Did you hit anything?
What make engine does this thing have? Tecumseh Briggs etc?