I am currently at the end of this same problem. I can get it to start one-in-a-hundred tries, but 99% of the time it spins once and stops. Without the plug, the starter can turn it over and blow fuel through the hole. It will only go up to about 60psi and gets stuck. Solid spark (new plug and wire). Starter bench-tested ok (but that's no load). Solenoid tested good. Good voltage to the starter so wire and ground tested good. To get enough oomph to start the engine, I had to jumper the starter straight to my big truck battery and give it a solid 14V to spin it fast enough. Even a brand new UTV battery didn't have enough oomph to do it. With everything else ruled out, I have a new starter on the way. Will gladly report back if that solves the issue for me.
*In my scenario, this was immediately after an engine rebuild so I KNOW the internals are good*