[73 Mustang Boy]

The backfiring probably does have something to do with the carb. I am going to run through how to adjust everything in one fatal swoop so hold on. First, unhook and plug the vacuum line going to the distributor. Then if you can use a vacuum gague to set the timing for the highest vacuum reading. If you dont have a gague just set the timing arround 12deg (thats usualy a good starting point). Then go down the page and look at the post titled "stalls when gassed" follow the instructions there on how to reset the idle mixture screws. Then reset your idle speed to get a good smooth idle (probably arround 700). This should take care of the problem, you may not feel as much power with the timing set lower but 25-30deg is way too much.