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vacuum advance

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zimmy

zimmy

Jedi Knight
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could someone explain the mystery behind the
vacuum advance?
just what does it do?
how can i tell if it works?
and if i get a petronix ignition
will i still need it?

i just dont get it! ........z
 
The vacuum advance advances the timing by pulling the plate that the points mount on when the engine hits a certain rpm range. To check it put your finger on the opening where the hose goes. Push the lever in. If you can feel suction on your finger when you let the lever go, it works. Yes you still need it.
 
ok, if it wasnt working, would it affect
your gas mileage alot?
ill check mine tomorrow...im car'd out
today already ........z
 
You'd probably notice it more in the way it runs. If it feels like it has no umph, that could be it.
 
Gas mileage would be the only thing it would really effect unless it was causing a vacuum leak giving you a lean run condition. The vacuum advance only works at part throttle/high rpm (cruising).
 
Just to add a bit to fabmandan's post....

The function of the vacuum advance isn't entirely predictable by RPM. The advance functions in response to one of two vacuum sources, ported vacuum (from the carb venturi) or less often... from manifold vacuum. The more common application is ported vacuum which reaches a peak during part throttle cruising. This part throttle advance of the timing results in better fuel economy. So, in answer to your question "would I miss it", you would not notice a performance decrease without the advance but you may see your fuel economy drop a bit if you normally spend a lot of time at part throttle.

If you buy a Pertronix, I'd leave the vacuum advance as is unless you have some specific reason to disable it.
 
It's instructive to hook up a timing light, connect a mighty-vac to the line to the distributor, and watch what happens. You don't get any advance until you have a few inches of vaccuum--I forget exactly where it kicks in, but this stuff is well documented on the web. You can have plenty of vaccuum at the carb port, but there still can be a problem with the advance mechanism in the distributor, which could prevent the advance from working. This is what happened to me. The distributor's vaccuum diaphragm thingy (pardon the high-level technical terminology) was frozen. Fortunately, it is available and not too expensive.

I didn't see any real difference in mileage or performance after I fixed it, but I don't do much highway driving, either.
 
Zimmy, its not just vaccum. There is a centrifigul slinger in there as well that advances the timing. many times this get all crudded up and does not work. You need both working to get the proper power and all. We had a morris van in england that had a really messed up distributor and ended up using a manual choke cable to manually move the distributor (like an old indian motorcycle) and this woorked pretty fine. Pushed it in for idle and then pulled it out for highway.
 
Kim, manual timing advance... that's Model-T era drive by ear stuff!
 
Just to add to the confusion, I think it was the '78 or '79 that had vacuum RETARD (no, not that kind of retard, the opposite of advance) on the distributor. Somebody here was telling me about it, but when I looked at the specs in the service manual I confirmed that was indeed a vacuum retard.
 
For what it's worth, Jeff at AdvanceDistributors does an AWESOME job at cleaning up and setting the advance on your stock dizzy. He also has great prices on the Pertronix if you don't have it yet.

He completely rebuilt mine to better-than-new and put in the pertronix for less than $200 (if I remember correctly). I am second guessing myself now. I think it was like $186 or something? $286 sounds like too much.

Keep in mind, those springs are decades old, and in my case, VERY stretched out. I had a ton of play in the mechanical weights, and the shaft even had some (not too horrible) play in it.

I was hesitant to drop that cash, but I am SO glad I did. The car runs a whole lot better!
 
i can get a new dizzy from moss with
the electronics in it for less than
$200.....

btw....how do u set the vacuum advance?
whats the screw thing do? ......z
 
The vacuum advance is "fixed" it is not adjustable. The thumb screw is for fine adjustments to the overall advance.

A generic distributor will have a pretty conservative curve. Jeff can set the curve to your exact engine's needs.
 
So, what if you have the origional distributor and it was not hooked up to the carbs will it effect the running of the motor? or is is not really necessary? Seems mine was purposefully disconnected. I am not to the point of running the motor.

Mark
 
The only issue with leaving the vacuum advance disconnected is there may be some minor scatter in the timing as the breaker plate may not have any preload on it. An acquaintance of mine in OZ puts a long screw through the breaker plate so it won't move. This reduces all the advance to centrifugal but it also eliminates any plate movement. Oh... don't forget to plug the vacuum line.
 
zimmy said:
i can get a new dizzy from moss with
the electronics in it for less than
$200.....

btw....how do u set the vacuum advance?
whats the screw thing do? ......z
Click to expand...

Zimmy, before you do anything big at least talk to Jeff. He'll readily explain why the new units aren't desirable. I was going to go that route too, and did the rebuilt. For what it's worth, the rebuilt looks as good as new and functions better!
 
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