Another 5.3L won't start with 6014 MSD box

[aviatorskate]

Trying to complete first start and break in of a carberated 5.3. Engine cranks but backfires thru carb and exhaust, won't start. 6014 box is communicating with the laptop. I can see engine rpm, voltage too the box. Things I've done;

1. Adjusted map sensor to all Zero's.
2. Adjusted ect inputs all Zero's.
3. Swapped both coil pack harnesses.
4. Sent all data to box and selected custom 1 on box.
5. Double checked and triple checked all msd harness connection's.
6. Plenty of fuel in carb.
7. Only error I get in software is ect input, It's not connected right now.

I am getting spark because it's backfiring.
Any help would be appreciated before I pull timing cover off and check timing marks.

Thanks
Craig