Flywheel Only Turns With Spark Plugs Out?

I have a Sears riding mower with a 12hp Briggs and Stratton engine. It was working fine. Now when the starter engages the flywheel it spins 3/4 and stops. The drive belt is off and the blades are off, so there is no bind. I take out the spark plug and it spins normally. When I put the plug back in it does the same, it turns for 3/4 of the way and the flywheel stops. Any suggestions?

By Daniel Garza from Odem, TX