Do I understand that it cranks over properly with the spark plug removed, but not with the plug installed. correct? What is the engine brand and model information, please?
I think a valve adjustment should work wonders for you, so let's start there.
Intake - .003"/.005"Exhaust - .005"/.007"The setting should be done on a cold engine with the piston 1/4" past top dead center on the compression stroke.Here's some additional information that shows the process in detail. Scroll down to the bottom of the page to where it talks about adjustments. It'll link to some helpful pictures as well. If there's anything you're not clear on, please let me know.Valve InfoHope this is helpful! Let me know and we'll continue if this doesn't solve it.