, it would be best if you began a new discussion and title it "Backfiring II." Then supply all the specs on the engine - year, cubes, heads, compression, intake, carb, exhaust, ignition type, and any other modifications that may have been done. Rebuilt or not. What octane gas do you run. 4-speed or automatic. Rear gearing. Are you at sea level or at high altitude like Denver (this does make a difference).
All of these things may not seem important, but they are in knowing what your combo is and then matching your timing and advance curve to the engine/car.
Start from scratch with the info and then help can go forward from there. Then do what is suggested and provide results so your situation can be dialed in and resolved.