I have a 1963 Impala with a 409. It has one 4bbl. Every time I add gas to the car it immediately acts like it's running on 6 cylinders. It bucks, surges, doesn't want to idle, and this goes on for about 20 miles before it starts running 'normal' again. The car will run normal until I add gas again. The inside of the exhaust pipes is sooty black indicating it is running rich? I would appreciate any help. This is driving me crazy.