Will the warmed up engine crank if you use a 12V boost? It sounds like something might be binding up inside the engine like pistons fitted too closely the the cylinders. Is the engine freshly rebuilt, or does it have some miles on it?

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!