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Old 02-02-2005, 02:28 PM   #1 (permalink)
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Dual vacuum advance distributor

I have maybe a dumb question. I've got a 1970 302 that is all stock, and it came with the dual vac. advance distributor. This is a daily driver that I installed a Pertronix kit. I currently have the advance hooked up from manifold vacuum to the front of the chamber only, the back of the chamber is plugged. My question is, can I replace the dual vac. chamber for a single chamber on this distributor? I noticed the dual chamber moves the advance plate very little like 1/4 inch, but a single chamber will move the plate quite a bit more.
If you look up part numbers for a vacuum advance it askes what distributor number you have. So is there an internal difference in the distributors? Thanks in advance.
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Old 02-03-2005, 06:33 AM   #2 (permalink)
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Dual vacuum advance distributor

first I'd like to say that 1/4 inch rotation on the dizzy is alot of degrees of advance. Just how much advance are you trying to run? Whats your base timing and how much mechanical advance does your dizzy have? How much vacuum does your motor pull? You might not have enough vaccum to pull the vac advance plate all the way.

I've never seen a dual vac dizzy in person so I cant say if it uses the same vacuum modular mounting bolts, same lenght adjusting arm, and rotating connection on the internals as a single vac dizzy. But I'd bet you should be able to put a single vac can on it, but I also dont think swaping it would allow any more vacuum advance than you already have.
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Old 02-03-2005, 11:08 AM   #3 (permalink)
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Dual vacuum advance distributor

Dual advance was used to temp control the amount of advance, and a thermal switch was usually mounted in the intake behind the stat. It was used to control the amount of advance during cold start and warm up.

Hooking your advance to manifold vacuum is a waste of time and defeats how vacuum advance is supposed to work. You might as well unplug it and just advance the mechanical.

Do a google on vacuum advance for a full tutorial. Many websites explain it fairly well. Most hot rodders just go with a re-curved dizzy and straight mechanical. It's a little easier to figure out.
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Old 02-03-2005, 12:30 PM   #4 (permalink)
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Dual vacuum advance distributor

The reason I ask is, I have a good single port vacuum chamber that I installed and when I tried to start the car, it wouldn't start at all. It acted like the timing was way off. Reinstalled the original and it started right up. The length of the arms that attach to the advance plate are the same length, but when vacuum is applied to both chambers the single port moves alot farther. So I'm thinking the advance plate is moving too far with the single port. Also the ported vacuum switch is not there anymore to control the dual port. Which is the reason I thought I would install the single port chamber. I've never had any dealings with dual chambers before. When I looked up part numbers for vacuum advance for different Fords, they ask for part numbers off of the distributor to get the right vac. advance, which made me think that there where some internal differences in the distributors. Thought somebody here might have run into this before.
Maybe I should run the single chamber to ported vacuum instead of manifold vacuum, then maybe the advance won't come in till after the car starts?
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Old 02-03-2005, 12:59 PM   #5 (permalink)
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Dual vacuum advance distributor

A dual port vaccuum chamber can work just like a single. One port was used to retard the timing and the second port advances it.

All you need to do is eliminate (just remove and plug) the vaccuum hose from (I believe) the inner port. Then hook up the other port directly to a carb vaccuum source. I have always preferred using ported vaccuum.

The vaccuum tree may be required to pass an emissions visual test, but otherwise is not needed.

Next check your total timing at say 3000rpm with both the vaccuum applied and disconnected. This will tell you if you are advancing properly. You can then try the same thing on the opposite port (in case I have my ports backwards.)

I have never tried to convert one to a single port but would assume that timing would change.

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[ This Message was edited by: dennis111 on 2/4/05 1:02am ]
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Old 02-03-2005, 02:03 PM   #6 (permalink)
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Dual vacuum advance distributor

A couple things... first dennis111, if you 'plug' the rear port, would that make the vacuum diaphram unable to move because air is trapped in the rear chamber? I haven't ever taken one apart so I don't know, but it seems like the best thing to do would be to just leave it open to the atmosphere instead of plugging it.

Also, there are two places you can hook the vacuum advance, to the timed vacuum port on the carburetor, which only gets manifold vacuum after the throttle is cracked open off idle, or just hook directly to manifold vacuum. I've heard many arguments for hooking it to manifold vacuum, such as reduced idle temp, reduced emissions at idle, etc... but I think it's easier to tune and get a consistent idle with it hooked to the timed vacuum port on the carb.
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Old 02-03-2005, 03:34 PM   #7 (permalink)
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Quote:
On 2005-02-03 14:03, Motorhead wrote:
A couple things... first dennis111, if you 'plug' the rear port, would that make the vacuum diaphram unable to move because air is trapped in the rear chamber? I haven't ever taken one apart so I don't know, but it seems like the best thing to do would be to just leave it open to the atmosphere instead of plugging it.
Perhaps I wasn't clear enough but I will say it again (because I don't know how else to say it. Plug (or remove) the rubber vacuum hose. You do not need to do anything to the cannister itself-I never did.

Quote:
Also, there are two places you can hook the vacuum advance, to the timed vacuum port on the carburetor, which only gets manifold vacuum after the throttle is cracked open off idle, or just hook directly to manifold vacuum. I've heard many arguments for hooking it to manifold vacuum, such as reduced idle temp, reduced emissions at idle, etc... but I think it's easier to tune and get a consistent idle with it hooked to the timed vacuum port on the carb.
I prefer using the "timed" vacuum that occurs off idle. I think it gives me a more consistent idle, especially at initial startup.

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[ This Message was edited by: dennis111 on 2/4/05 3:38am ]
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Old 02-03-2005, 04:00 PM   #8 (permalink)
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Ah sorry, I interpreted what you said as remove the hose and plug the canister, not remove the hose and plug the hose. My mistake.
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Old 02-04-2005, 12:07 AM   #9 (permalink)
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Dual vacuum advance distributor

When I got my '68 302 it got 8 MPG in a '68 Galaxie. It also ran like a dog, hardly moved off the line, and didn't have much oomph up top either. My '67 Gal with a 289 and the same drivetrain got 14-15 MPG typically and was rather quick, blew away the '68.

Just going from the dual vacuum advance/retard unit to an adjustable advance unit got me up to 12 MPG. Not a single other change to the car. It also worked wonders for acceleration. It could actually get out of it's own way! After advancing initial timing a little too and rebuilding the carb and a whole tune-up I managed a high of 17 MPG highway and 14 around town. Timing is not everything, but it's damned close!

The purpose of the advance/retard unit was to control emissions. It also has a side benefit of if the car was overheating it would switch the advance vacuum from ported to manifold to give more timing at idle, raising the engine RPM and thus pulling more air through the radiator helping to cool the engine down. Not sure how much it actually helped but I do know that on the couple occasions I've overheated simply raising the RPM a little cooled it back down pretty quickly. The more timing the cooler an engine runs, the less timing the hotter it runs (since more energy and burn time of the fuel is wasted).
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