Holley 750 backfire.

the primaries are too lean....stock 750 is usually around 70 .....you can go to holley web site and check to see what the stock jetting is....

also..i would have the same jets in the primaries with a single plane intake....not 68 and 65....

also check your squirter size....at least a 31 ........

and finally check to make sure as soon as the throttle is moved that the accelerator pump squirts gas...you might need to adjust that....