Locking up

[builderpete]

I am not a mechanic just someone who loves playing with small engines. I have a craftsman / powerbuilt 15.5 500cc i/c engine. I have just put a new / used connecting rod on. The camshaft was in good order. but the shear pin to the lower spindle on the flywheel was broken and the flywheel was 30% out of line. I replaced the shear pin . Now when I turn the engine over without a spark plug the crankshaft spins at a good speed. When I replace the spark plug the engine spins and stops in the same place. Am I looking at valves, timing or what? Any help will be greatly appreciated . Remember I'm a chancer so explain clearly please.

 

[ILENGINE]

Lets start with the engine model type and code so we can see what engine you have. At this point we don't know if you have a flat head, OHV, OHC, or some new fangled aerospace valve system.