Set timing to factory specs.
Check your points, cap, condenser. The condensers available today are all absolute crap from China. They fail in a few hundred miles if they work at all. You should get one of the 200,000 hr condensers made by one of the members of this forum.

https://www.stovebolt.com/ubbthread...55045/the-newest-condenser-update-8-2024

What carburetor do you have? Rochester B? Carter YF?

I had backfires through the carburetor at idle before I rebuilt a proper Carter YF. It was the accelerator pump leaking. Rochester carbs are prone to warping causing the accelerator pump to leak.

1950 Chevrolet 3100 (Ol' Roy)

1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)