The Stovebolt Site Forums The Parking Lot Big Bolts won't idle, backfires through carb

It sounds as if it may be retarded you may have to bend that steel vacuum line a little.
Also how dose the compression seem with the combustion camber being in the cylinder in the 409 it is common for the rings to get damaged by carbon. If you rebuild this motor and have it bored be sure the machinist puts the lip back in the top of the bore for the combustion chamber or your new top ring will drag carbon down into the cylinder casing premature failure.