Hello there. M900 Refurb

Hmm this is turning into a mystery. I have entirely bypassed the starter relay and cable by connecting a powerpack straight to the starter motor so that eliminates relay and cable issues. I have managed to borrow a brand new starter, so that eliminates that. The engine turns over easily by hand and I have double checked the cam timing and still it struggles when turning over. At a loss I pulled the flywheel off to see if anything was amiss but all looked fine.
So I'm stumped. Next option is to drop the engine back out and strip and start again....