Backfiring out carb and wont run right

I would recheck float levels, making sure the needles and seats do their job to stop fuel from dumping in. Adjust accelerator pump and get initial timing where it wants to be like aengingguy described. My 440 wants 20-21' initial, cams with 110' lobe separation want more initial, I then had to recurve distributer down to 10-12' mechanical to give me the 32-34 total I needed. Before reworking timing I had same problems as you. Do one thing at a time so you know what's working. In my case, my stock six pack set up was way lean for the .040 over 440 with match port eddy heads and aggressive cam. (Backfire through carb and spit all over). Timing dial in, big bump in jetting, vacuum secondaries and floats set correctly, and acc pump shot increase and she's starting to run like a raped ape. It's been a long slow road for me since I'm not a professional mechanic, a lot of reading and research but when it starts to come together it all makes sense. Good luck