Back fire through carburator. New sbc engine. New everything

Back fire through carburator. New sbc engine. New everything

Upon startup it has backfired through the carb. It's a 355 all done right. This is not our first rodeo. It's a hyd roller cam, New 600cfm holley all new everything. Need to diagnose this pop. It is not the timing. We have been through that several times already.