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geezer#99 · 05-30-2014, 10:31 AM

To clear a couple things up there's only one distributor vacuum advance. Uses manifold (sometimes called full time) vacuum to operate it.
The vacuum is supplied to it from the carb. The supply ports on the carb are either manifold or ported ports. Normally any port below the throttle plates is manifold and any port above the throttle plates is ported.
The manifold port supplies vacuum all the time. The ported port supplies no vacuum when the throttle is closed fully (like at idle). If you follow the ported passage it goes into the venturi above the throttle plate. The throttle plate acts like a valve, shutting off the vacuum which is present below the plate.
So at an idle your vac advance pot can receive manifold vacuum and advance your timing or receive no vacuum from the ported port.
Which to use is decided by how your motor is set up. A stock motor (or slightly modified) that idles nicely at 600 to 700 rpm could use ported vac to the distributor. Slightly modified means a better intake, carb, headers and an rv type cam. When you move up a step to a more radical cam, bigger carb, higher rpm type intake and bigger headers then you might need to use manifold vacuum.
It all depends on getting your motor to idle with the throttle plate closed enough to keep the transition slot in the venturii covered. The transition slot is the port that supplies the motor with fuel so it will idle when the throttle plate is closed. If the throttle is open too far, the transition slot is completely uncovered and stops supplying fuel and the carb starts to work on the power circuit and supplies too much fuel and idles rich.
In order to get your throttle closed back up you need to speed up your motor by giving it more timing. Manifold vac to the vac pot will do that. When your idle comes up you can slow it back down by closing the throttle plate, covering the transition slot, making your mixture screws work again and not idle rich.
So which port to use all depends on your set up. Experiment. See what your motor likes.
Keep the throttle plate closed, your mix screws working and your motor idling nicely.

05-30-2014, 10:31 AM	#9
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,777	Re: Vacume for HEI distributor To clear a couple things up there's only one distributor vacuum advance. Uses manifold (sometimes called full time) vacuum to operate it. The vacuum is supplied to it from the carb. The supply ports on the carb are either manifold or ported ports. Normally any port below the throttle plates is manifold and any port above the throttle plates is ported. The manifold port supplies vacuum all the time. The ported port supplies no vacuum when the throttle is closed fully (like at idle). If you follow the ported passage it goes into the venturi above the throttle plate. The throttle plate acts like a valve, shutting off the vacuum which is present below the plate. So at an idle your vac advance pot can receive manifold vacuum and advance your timing or receive no vacuum from the ported port. Which to use is decided by how your motor is set up. A stock motor (or slightly modified) that idles nicely at 600 to 700 rpm could use ported vac to the distributor. Slightly modified means a better intake, carb, headers and an rv type cam. When you move up a step to a more radical cam, bigger carb, higher rpm type intake and bigger headers then you might need to use manifold vacuum. It all depends on getting your motor to idle with the throttle plate closed enough to keep the transition slot in the venturii covered. The transition slot is the port that supplies the motor with fuel so it will idle when the throttle plate is closed. If the throttle is open too far, the transition slot is completely uncovered and stops supplying fuel and the carb starts to work on the power circuit and supplies too much fuel and idles rich. In order to get your throttle closed back up you need to speed up your motor by giving it more timing. Manifold vac to the vac pot will do that. When your idle comes up you can slow it back down by closing the throttle plate, covering the transition slot, making your mixture screws work again and not idle rich. So which port to use all depends on your set up. Experiment. See what your motor likes. Keep the throttle plate closed, your mix screws working and your motor idling nicely.