Vacuum advance questions

[TulsaSpider]

"if you're not going to use the vacuum advance, you need to modify the centrifigul weights for more advance"
Mark you posted this on a recent thread. I didn't want to hijack it.
As has been stated before the vacuum advance is expensive to replace, goes out quickly, and is only used during partial throttle. So Mark, or whom ever want to chime in on the subject. What weight would you recommend and where to get them?
Thanks
Clark, snowbound in Tulsa

[manoa matt]

If you turn it upside down and look at the plate the advance weights are mounted on you will see a post that drops down into a slot. You either need to file the slot so the post can advance more, or file down the post.

In order to do so, you need to remove the central shaft from the distributor = complete disassembly.

FYI you are not modifying the weights or springs themselves. Modifying the weights and springs controls the advance curve. You are modifying the advance limit of the distributor.

The vac module advances the plate the electronic pickup is mounted on and only works during periods of high intake vacume, ie. idle and part throttle. At wide open throttle there is not much vacume and hence not much vac advance.

It also depends on the distributor you are working on. There are two versions of the 79-85 electronic distributor. One had 22 deg of centrifugal advance and one had 28 deg. You can verify if you remove the rotor and move the weighs out of the way. the 22deg version will have 11 stamped and the 28 deg version will have 14 stamped into the advance plate.

I would not modify the 28 deg version. If you set your initial timing to 10 deg and add 28 you end up with 38 deg total advance which is plenty. If you modify the 22 deg version you only need about 4 deg more advance so your total advance is 36. Or just you set your initial advance to 14.

The Marelli Plex distributors are the same basic unit minus the vac advance. They are purely mechanical advance and 28 deg. units.

[majicwrench]

Nothing wrong with keeping the vac advance, and you will get better mileage. I would be nervous about changing my dist curve with out verifying with a timing lite or a good old fashioned dist machine.
A safe bet would be getting a dist that came without the vac advance.
38 degrees is about the proper amount of advance for accelerating, and the vacuum advance does not make any diff when accelerating or at full throttle. But when cruising, you can advance the timing way past 38 and improve mileage . That is what the vac advance does.

[So Cal Mark]

Matts' write up is very good and exactly what I suggest

[TulsaSpider]

The problem with the vacuum advance units is they are expensive and from what I have understood, fail quickly.
I hope this helps not only me, but anyone who has a Fiat electronic unit. Thanks!

[backyardbill]

Has anyone modified a vacuum advance unit from another car, like an old Chevy or Ford?

I took my broken unit apart, and the bellow was hard and cracked. It looks possible to make another bellows from some material like innertube rubber or something like it.

[majicwrench]

I have repaired the diaphram on choke pull-offs (the same basic idea and construction) before, so I know it is possible.
Keith

[So Cal Mark]

whatever diaphragm you use is going to be subjected to the heat from the exhaust manifold and fail prematurely

[manoa matt]

The stock vac advance unit will be calibrated or built to a specification to give a certain degree of advance per a given vacume value. If you re-make/repair the diaphragm how will you know what advance it gives?

There were also 2 different vac units. One gave 10 deg advance and another gave 14 deg. I have now way of confirming this, but I would guess the 14 deg unit went with the 22 deg advance distributor, and the 10 deg unit went with the 28 deg advance distributor. So both distributors would give a total advances of 36 and 38 respectively.

A more elegant and reliable solution would use modern technology via a map sensor combined with circuitry to control the advance similar to the aftermarket 123 ignition distributor or Megajolt Jr EDIS sytesm.

[TulsaSpider]

Ok I went and got my dizzy out and I cannot discern any numbers that were before spoken of, where are they again?

[majicwrench]

Per Matt's post, repairing/replacing the diaphram is not going to change any calibration. The spring that pushes against the diaphram and the length of travel are what need to stay the same.

[TulsaSpider]

I see a 16938, and Magnetti Marelli of course stamped... Make that 16 (degree mark) I could find no other markings?

[TulsaSpider]

Matt?

[manoa matt]

Tulsa, Remove the rotor, remove or push the advance weights out of the way. On the metal plate the advance weights are mounted on is where the number is stamped.

[TulsaSpider]

Yup did that. Mine has 16 degrees stamped on it. It came out of a carb'd 79 2L so I guess I'll just perform the mod.