get the timing set right first, next rebuild that carb, it sounds like its most likely your problem, make sure to give it a proper soak in carb cleaner for a day, and then use the spray carb cleaner to blast it down after taking it out of the soak.... let it dry and assemble... i dont know neccisarily how difficult that carb is to rebuild though, i'm not a GM guy but that should get you in the right direction