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I just replaced the belts on this mower. I had the mover on its side while I
completed the work. I turned it over to test and the pull cord will not
budge. Unit has 3/4 full oil. even if I remove the spark plug it
still will not turn.

Any ideas.

Thank You

Parasitic load maybe.
Something to do with the fitting of the belts.
Remove the belts and see what happens.

I would check to see if the belt is stuck between the pulley and belt guides.