17 hp Briggs motor hangs on compression

I have a 17 HP Briggs & Stratton Engine model 31C777-0173E1-0211052E. The motor spins until it hits compressions then stops. Sometimes you can turn the motor "by hand" 1/2 a turn and the starter will spin it until it starts. Once started, it runs good. We have replaced the starter and checked the wiring and connectors. We have set the valves several times from 003 to 007 from TDC downs 1/4 inch. We did a compression test "when it would spin". It shows 90 psi on gauge. It will spin freely when spark plug is out. When you put the spark plug back in, it hangs on compression again. We also checked the flywheel key. it was good.

Any ideas on what to check next to make it run properly?