I own a 1991 305 tpi 5 spd. manual and im having issues with backfiring. Ive set the timing anywhere from 0 to 6 degrees before tdc and get same results and ive made sure to disconnect the wire, replaced spark plugs (noticed there was blow by on the plugs) and wires, cap and rotor as well. Ive checked for vacuum leaks and found one, fixed it and have not heard any leaks yet. What is going on?

 

Your diverter valve is bad...