[Purdy Swoft]

A vacume leak can cause the symptoms that you describe The leak could be between the manifolds and the block or where the carburetor bolts to the intake. If you are running one of the potmetal carburetors, they are famous for warping. A vacume leak causes the engine to run lean. Opening the GAV richens the mixture and helps compensate for a lean condition such as a bad gasket or warped carburetor mounting flange.. Most inline filters are for use with a fuel pump and could restrict gravity flow. restricted flow causes a lean condition which usually results in backfire. Black sooty plugs can mean several things . Air filters, too cold spark plugs, weak ignition will all cause sooty plugs. Points gap can be adjusted at anytime without messing with the timing. You said that you adjusted the points gap and reset the timing. You say that it backfires. Backfiring is also a symptom of timing being incorrect . If the timing was good before, It shouldn't need resetting unless the points cam had been loosened and moved . You could have timing problems .