I know I;m coming in late here... But I think you're feeling that "the car was set up for manifold" is probably mistaken. If your engine is basically a 440 with a cam, in most cases it will run better with the timed vacuum port connected to the distributor. Not always... but most of the time by far. Now, CP noted how the timed port works. At idle, if the engine is tuned properly, the throttle plates of the carb are closed just below the passage that conencts to the timed vacuum nipple on the carb base. That means at idle, there is no vacuum present going to the distributor. However, if there are other tuning issues.. not enough initial timing, or poor idle mixture, or too fast of an idle, the throttle plates are opened using the idle speed adjustment an that port gets exposed to vacuum at idle. That can really screw things up. If it were me, I'd take that carb, set the idle speed and mixture with the vacuum ports plugged, then connect the vacuum advance on the dsitributor to the timed vacuum port on the carb. If the idle speed goes up at all when you connect the vacuum line check the timed vacuum port on the carb to see if there is any vacuum (suction) present while the engine is idling. If there is, slow down the idle, add about 4° of timing, reset the mixture screws to smoothest idle, and make sure there is no vacuum at the timed vacuum port after you're done.
