Starting about a month ago my 1976 F100 w/ a 302 and a 2 barl Mortorcraft carb with a vac adbance Started backfiring, when I try to accelerate or punch the gass form a idle. I've rebuilt the carb, changed the spark plug wires one by one, and tooned and timed the crap out of this damn thing. No mater what I do It will still backfire.

 

Sounds like a lean condition. Either the accelerator pump isn't working, there's a vacuum leak, or both.

 

Have you checked your timing? Sounds like it may have jumped a tooth on the chain.

 

How does it idle? It could be a burned intake valve. Was the cam changed recently? Cam going flat will do the same thing.

 

the idle is kinda slugish and if it dosent get a chance to warm up it will die. I've checked the timing several times to no luck. But it is running a little smouther sice i changed the motor mounts. I'm going to try the timing again today and see if that will make a change.

 

This is what I am thinking also

 

so yester day I retimed it and it ran pritty good idling from 640 to 680. but during acceleration if i try to accelerat to fast and give it to much gass it will backfire still and it dosent matter where i have the fuel mixture set to.

 

sounds like the accelerator pump is shot, as mentionted before, or the vacuum advance is not working