Random Backfireing? Help.

could be a few things, but i once bought a charger that i had to dig out and get running. i used a 2 gallon gas can to feed the fuel pump. i got it running real good. then i switched to the tank feed and poured the rest of the can fuel into the tank. after about 15 minutes it started to pop out of the carb every few minutes. i drove it home and dropped the tank and the fuel was really dirty and the tank was rusty. i installed a fuel cell and replaced the entire fuel system and threw a holley 600 on it, and it has not done it since. i have heard of bad gas doing this, but not right out of the pump, although im sure it is entirely possible. i would get some gas from somewhere else , run it out of the can, a new filter, and try a different carb just to rule out a fuel problem. my $0.02