R53 won't idle after rebuild
 

As the title says, I'm having some issues getting this fresh engine to run correctly. If I hold the throttle a little it will run around 1500 rpm. If I let it go the engine will stall. My son also noticed the header was turning red hot so I shut it down.
Compression test shows 150-153 psi on all cylinders. Plugs looked all sooted up when I looked at them after the engine cooled down.
There is no check engine light on when it was running so I can't pull any codes.
I'm at a loss, any ideas/ help would be greatly appreciated