Holley 750 backfire.

I just recently bought a Holley 4150 hp series 750 from a local carb guy. it's been rebuilt and I was told should perform well on my mild 446. It takes of fine but as soon as I get on the throttle it starts to spit and have a back fire. Timing is set at 38* a 3000k. Running a single plane tm7, 509 cam,3000k stall. Stock Mopar electronic ignition. The ecu is actually off a 360 engine setup could that be a problem? Took the carb apart to see jetting, primary is 68 65. And secondary is both 80 jets. I'm running a 6.5 pw also. Do the primaries jets seem small for a 750dp hp series carb? Thanks in advance!!