backfire through carb, need help

[88Stanger]

I am almost embarrassed to ask this, but i really am at my whits end here. Here is what i have, new 383 stroker motor, full roller, Edlebrock aluminium heads, new msd distributor, msd 6al box, Fitech fuel injection, initial timing set at 12 degrees and total at 34 degrees. motor starts right up and idles great, slow roll into throttle, it runs great, stab the throttle, and poof up through the carb, sometimes even killing the motor. i have adjusted the valves several times, check timing several times and still this issue exists.
Any help here would be great.