GM-GMC: trouble shooting, spark plug wires, gmc 1500

Question
I have a 1996 GMC 1500 series 4x4 pickup with a 4.3 gas engine fuel enjected automatic transmission. I recently had a tune up ( plugs,wires,cap,button,air filter,)After these parts were installed and I left the garage the truck started to backfire through the carb only under a strain. We changed distributor,noxsencor and even put the old cap and wires back on and we still have the same backfire through the carb, What else could I check, PLEASE HELP signed ROB  

Answer
Make sure that the spark plug wires are correctly connected and in the correct firing order.

After checking the wires, try spraying a little carburetor cleaner or WD-40 along the areas where the intake manifold meets the cylinder head.  Any change in engine speed indicates a leak of air into the the intake and this will make the cylinder(s) fed by that area of the intake manifold run lean and tend to backfire.

If the above test does not cause any change in engine speed, the vehicle should be taken in to a reputable shop for a diagnostic check up.