Vacuum Advance - Manifold or Ported

[SuperHart]

MaSnaka, on a Holley carb the manifold (direct) vacuum nipple is located on the carb baseplate BELOW the throttle plates. The LARGE nipple on the base plate is used for the PCV connection. Ported vacuum nipple is located usually on the side of the primary metering block above the idle mixture adjustment screw. Another way to tell is to unplug the vacuum nipple during idle. If the idle speed drops and becomes quite rough and then smooths out when you replace the plug, that is usually a manifold vacuum nipple. Manifold vacuum at idle draws the diaphragm inside the canister forward which advances the timing (assuming that the distributer rotates counterclockwise). When manifold vacuum drops the spring inside the vacuum advance canister pushes the diaphragm away causing the advance to disappear.

[Excaliber]

On the typical vacuum ADVANCE unit, the more vacuum the more advance. It's straight forward like that, no vacuum, no advance.

However, on certain model cars (pre electronics generally speaking) a vacuum RETARD was also employed. Often it's a 'dual vacuum' diaghram, one side advances and one side retards, each side has a vacuum line attached. Vacuum to either port was controlled by a myriad of mechanical, electrical or othe vacuum operated switches and ports, based on engine load, temperature (ambient, engine, inlet air, etc.). This type of system usually has a nightmare of vacuum hoses associated with it and was largely replaced with the introduction of computer control and EFI.

[MaSnaka]

Ok you guys I'm getting somewhere with all this discussion. I am getting my advance from the port nipple just above the idle mixture screw. I have my PCV hooked up to the large nipple at the rear of the base plate. I have 2 nipples side by side, 1 small and 1 large at the front of the base plate (both plugged at the moment). I assume the smaller one is the one I plug into for my manifold vac advance, what is the large one for? Auto transmission maybe? Another PCV perhaps? Remember both of these are at the front of the base plate. My car was originally (when I bought it) set up with the manifold vac advance but a mechanic told me to switch it to port. At the time he really didn't explain the difference or the reason why, just did it and that was that. In listening to all of your collective advice I think I will switch it back and see what difference it makes. But I would like to know what those other nipples are for. Thanks! I'm learning.