Backfire problems!!!!!

Check the little black ground wire in the distributor. Move it around and see if has an open circuit. This can causes part throttle back fire.

 

You might want to check the end play of the distributor shaft, it may have to much play. I had this problem on my 64 F100 y block points distributor. Also I would do a compression test, you may have some bad valve springs (if you are using comp cams).

 

try a can of sea foam. and with that said, my 59 elky with a 305 and 600 edlbrk. developed a back fire thru carb.went thru plugs,wires,dist the whole deal. ended up being a slightly worn cam lobe.good luck keep us posted.