First, check and adjust your timing. Fouled plugs?

What carburetor is on your engine? Is the engine stock?

Is the backfire out the tailpipe or through the carburetor?

Your carburetor may be leaking fuel into the intake or the mixture may be set too rich.

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)