'71 Olds w/ 455 engine

[Diligenterprise]

Had the occassion to change my carb (Rochester 4bbl) and backfiring through the exhaust 3 exploded mufflers worth. Due to the age and emergent situation I used a later model used carb base to intake gasket. This gasket is completely open exposing all 4 bbls (no passages, etc.) The previous gasket had material shaped like a pyramid and you could see the passage ways imprinted from the bottom of the carb. Would this totally open gasket make a difference? I can only get it to rough idle; black smoke as 1 or more cylinders don't seem to be firing. Changed the vacuum advance and trimmed 1/2 inch off vacuum lines to get tighter fit.