The symptom: The engine will turn over 1/4 to 1/2 turn and then will bind up. The electric starter may click. When turning the engine over by hand, the same thing happens.
The cause: Your petcock and fuel needle valves/o-rings have failed and fuel has leaked past them and into your engine, filling up the cylinders with fuel. When you hit the starter button, the high-torque starter damaged a few of the starter gear teeth.
The fix: The engine will have to be removed, the cases separated and the starter gears replaced. Also, replace the petcock, float needle valves, and float valve o-rings, to keep it from happening again.
Preventative maintenance: Replace your petcock, float needle valves and float valve o-rings if you suspect you have fuel leaking past the petcock. $100 in preventative maintenance parts will save you $1000 in repair costs.