Engine is a 350 with 4bbl edelbrock carb. headers and intake manifold. It was running fine and just started backfiring at 40mph. It got worse as I tried differant ignition timing even when I put it back at 13degrees btdc. where it had been. Now it backfires when I start it and at low speeds especially on excelleration. I took off valve covers and everything looked and sounded good.
I fixed bad connections at the coil but not at the other ends of wires. The vacuum advance may not be working because when I pull hose to carb. nothing changes. Because it got gradually worse might be a clue for someone more knowlegable than this trial and error restorer.