help with engine backfiring
 

I need help trying to figure out an engine backfiring. I have an 82 sc with PMO carb conversion, and headers. plugs are looking good, wires are new. backfire is from the left side of the engine, not sure which cylinder. It does it when cold the most but continues when warm. Also does it when no load on engine at idle, and while driving down highway. I readjusted timing, seemed to help a little but it could be my imagination. Also, I don't know if this matters but the vacume advance tube on the distributer is not connected, I recieved the car that way and am not sure if it was due to the PMO carbs? any help greatly appreciated.