New motor backfiring through carb

Lets review the scenario, engine runs, remove from test stand,dyno or whatever.
Install engine in car, bolt on torque convertor or flywheel. I suspect engine was turned during some part of this, likely with distributor out. And likely distributor was out.
If crankshaft was turned over once or any number of times,on odd numbers then distributor could very well be out 180.