Dietbudda

So i've searched and can't find EXACTLY what i'm looking for.
I have a weird situation, I've installed the motor into the car, and when i try to fire it up, it won't catch and fire over.
I've got spark, i've got fuel and i belive i have compression. I can't get consitent readings with the tester i have.
Here's the weird part, pull the spark plugs out from the rear rotor completly and it will fire and run on one rotor. Put the plugs back in and the car won't start.
Is it a carbon build up issue?
Any advice will help. We've put some ATF fluid in the housings and are letting it sit overnight.
Thanks in advance.