85 back firing concern

i just recently picked up a 1985 GT, the motor has a holley carb, not sure of the size, an edlebrock RPM Performer intake, o/r h-pipe and a pair of headers. When driving the car, its a little loud and it back fires when decelerating. I suspected an exhaust leak and tightened up the nuts holding the down tubes to the header flange. That helped mildly but the car still backfires, now i'm suspecting timing or the section where the H is, is loose. If so, what are the stock timing specs, and if it is the exhaust, someone just let me know. I have the time and knowledge to fix it either way, i'm just out of ideas and cash, any help would be much appreciated. Thanks - Frank

i will defenetly check the timing first, and then maybe dive back under the car and rip the exhaust down or something to that effect. THe air tube couldn't be the problem merely for the fact that there is no air tube, and the bung where it mounts was brazed shut, so i don't think that could be causeing much problem. Thanks for all your help