Have a 1970's vintage crate motor (350/290HP) with headers & an Edelbrock 600 CFM carb (1406).

Motor had no issues until I started it after sitting in the garage for 1-year, runs rough & "backfires" through 1 side of the header exhaust (passenger).

Steps so far:

1. Drained all of the gas & refilled with premium, runs a little better, but still idles hard with a small miss & exhaust backfires loud upon deceleration (passenger side only). Sometimes while running at idle, it will also pop a little on the passenger side only.....

2. I tightened all of the header bolts, none of them were loose, & looked where a possible leak could be. Everything "looked" normal?

3. I pulled the plugs - they all are black. So I bought a new set, gapped them, but have not installed yet.

Am I on the right track to find the root cause of the exhaust backfire? What else should I check or eliminate as the root cause?

I was recently laid-off from work & want to get the vehicle running like a clock before Spring is upon us.....thanks in advance!