Carburetor - 66 fastback

[thunderbolt]

I am in agreement with Clark. Had that Holley, and while Holleys are known to be tempremental, mine worked great all the time. But backfires will blow out the power valve which will need to be replaced. I would start with a new cap and rotor. A cracked rotor builds up carbon deposits and starts arcing where it shouldn't. Give your ignition system a tune up and see what happens.