i have 79 bronco with the 400 it has the performer 400 manifold and a holly carb on it im having an issue with major back fires out of the carb when i snapp the throttle only,it also cuts my fuel if i snap the throttle ive pulled apart and inspected the carb and cleaned it but hasnt changed anything,i need some idea's i know im missing something im just to frustrated to think of it,also what should the timing be set at ?

 

Once the engine is warmed up, it shouldn't need choke anymore, so choke lean-out when opening the throttle plate abruptly shouldn't be an issue.
What about the accelerator pump not working, or the stroke on it set too short if it is adjustable?

I remember people years ago rebuilding carbs, and losing the accelerator pump check ball or slug when they took them apart and blew them out with solvent. Then they couldn't figure out why the accel pump wouldn't squirt!

If your float level is set too low, it could be sucking up air instead of fuel in the bowl when the accelerator pump reloads a pumps-worth of fuel.

I don't know the specific timing on that engine, but I would think that 10 BTDC would get you in the ballpark at least.