New motor now won’t start

A buddy and I have been working on a 2012 rzr 900xp. Got a reman motor from a forum sponsor and got it installed.

Monday- motor was in, oil and coolant filled and primed. Took some time to get it fired up due to a dead battery. Once it fired up it idled smooth for 30-40 mins as we went through the process of bleeding the cooling system. Idled around 1400rpm. Shut it down for the night without elevating idle. Plugged in a battery tender and left it be.

Thursday – installed the rest of the plastics and then again had a hard time starting. Seemed to only fire on one cylinder. Thought it was running decent and took it around the yard and half way back it felt like it lost power. Then it was a struggle to keep running.

Since then- installed new plugs. Verified spark and fuel in both cylinders. Swapped out possibly bad fuel. Ohm tested crank position sensor. Now will not start at all. Pulled intake and passenger side intake boot soaked with fuel. Crank motor and get a backfire on the pass anger side cylinder with a lot of air blow out. Occasionally with the right throttle position can get it to barely bang (very rough idle) on what seems like one cylinder.

Sorry for the long winded post but the best way to accurately diagnose something is with a lot of details.

Thanks a lot !!