4.3 backfire

im sure this has been talked about before but i cant find it. i have a 94 1500 4x4 4.3l tb.my problem is the truck is backfireing. i have chaned the fuel filter, cap and rotor, plugs & wires, air filter, ignition coil, checked fuel press., no visibale bad vacume lines, what else could i try. thanks for the help