Missing and Backfiring like crazy

[MandarinSixSeven]

Well i got my new carb and intake on, new plugs and wires...have it timed to 13degrees...and it is still missing like mad, never did this before, ive tried a couple different baseplate gaskets for the carb, still nothing. My buddy took the carb apart and the bowls were filled with crap, like shop dust and stuff and this is supposed to be a newly rebuilt carb, so i took it back and they put it on the scope and said i have 4 cylinders dead, but that makes no sense...my friend who took it apart said that it is most likely the idle jets and idle chambers are clogged with junk, which kinda makes sense to me, since after about 1600 rpm it smooths out. Hmm, what do you guys suggest i do, im gonna take it back tommorrow and get them to try a different carb to make sure, but im all outta ideas and patience