100 psi should be quite enough for it to start and run. You can forget about the carburetor, and intake air leaks, if it won't start when the chamber is primed with fuel. We are left with ignition or compression faults. I take it you are using the old spark plug that was already in the engine? Did you check whether that same spark plug would spark with its gap increased to 0.060"? Have you tried to start it with the muffler removed, in case it is blocked?
The spark plug might work at atmospheric pressure, but not work at 100 psi pressure. Hence the need to check it with a much larger gap.
