Help with Backfire

I just built a 360 for my Dart. Its basically a stock rebuild with a Hughes Whiplash, a Weiand action plus, and a Brawler 670 carb. I broke in the cam, then started the engine a few other times and everything was good. Today, I went for a short drive, and when I was pulling into my driveway, it started backfiring through the carb. Its about once a second, like a clockwork. I've tried advancing the timing till it was hard to start, and retarding it till it would barely run. I've richened up the idle mixture screws. I'm out of ideas at this point.