Vac Advance Unit, Testing Options

[76-914]

Is there a proper test for the dizzy vac adv unit? i.e., how many pounds vac and how long should it hold. Also, what about the retard side; the same?

[tod914]

If you hook your Mityvac up while the advance is mounted to the distributor, you'll be able to see if the armature functions properly. Check the gauge to see how many kg it takes to fully extend the arm. There's no point in adding more pressure than that. Leave it for 10 minutes and see if it drops. You should know right away if it's ruptured.

[orange914]

Do you want to check advance curve spec.'s or just vacuum test/spec.'s?
unfortunately, gone are the days when you could go down to a good shop and have your distributor's degree map checked and adjusted on a sun distributor spin machine. You are lucky to find one in existance and then finding someone who knows what it is or how to work it is even more rare! Nuff' ramble. Anyone have advance charts for the different 914 distributors?
Capn' Krusty, any input???

[Hammy]

I've never seen any numbers, I tried searching myself, and I believe there were a lot of different vac units used. Suck on the hose to check operation, plug and hold it with your tongue...works for me (IMG:style_emoticons/default/smile.gif)

[76-914]

When I pulled on the advance side(I refuse to say sucked) the armature would move but it would not hold a vacuum. I can't squeeze the trigger fast enough to produce a measurable reading and it won't hold with my tongue, either. The retard side produces no movement at all. So it sounds as if it might be dead. Next question; is this part still available?

[r_towle]

question.
Does the car idle ok when you pull the advance/retard hose and plug it?

You can drive the car without the canister...it just starts the advance faster or pulls the plates back (retard) sooner for emissions.

I am pretty sure (but I am certain I will be corrected if I am wrong) that the retard side only pulls the diaphram back from the advance position.
It sits at rest with no advance, so you would not be able to move it by applying a vacuum to the retard side....you would be able to hold pressure though.

RIch

[underthetire]

When I pulled on the advance side(I refuse to say sucked) the armature would move but it would not hold a vacuum. I can't squeeze the trigger fast enough to produce a measurable reading and it won't hold with my tongue, either. The retard side produces no movement at all. So it sounds as if it might be dead. Next question; is this part still available?

I probably have a couple good ones laying around. It should hold a vacuum for quite some time.

[Cap'n Krusty]

You're right, it's bad. Gonna be VERY difficult to find. Go by the number stamped in the arm, 3, maybe 4, digits.

The Cap'n

[Cap'n Krusty]

question.
Does the car idle ok when you pull the advance/retard hose and plug it?

You can drive the car without the canister...it just starts the advance faster or pulls the plates back (retard) sooner for emissions.

I am pretty sure (but I am certain I will be corrected if I am wrong) that the retard side only pulls the diaphram back from the advance position.
It sits at rest with no advance, so you would not be able to move it by applying a vacuum to the retard side....you would be able to hold pressure though.

RIch

Wrong. The retard pulls it down from the rest position, the advance pulls it up from the rest position. That way the idle is dropped with the throttle closed (timing retarded), and the timing advanced over time when the throttle is opened (adding to the mechanical advance). Neither side holds "pressure", only vacuum, which isn't measured in PSI, but in inches of mercury.

The Cap'n

[r_towle]

I know it holds a vacuum...
I did not think there was possible movement past the at rest position...and I knew you would set me straight..

Rich

[tod914]

I had wrong vac. units in cars already. Be sure to cross reference it. The tech tips 700? lists which ones goes with which distributor. Rich at High Performance house might have a NOS one for you.

[76-914]

question.
Does the car idle ok when you pull the advance/retard hose and plug it?

You can drive the car without the canister...it just starts the advance faster or pulls the plates back (retard) sooner for emissions.

I am pretty sure (but I am certain I will be corrected if I am wrong) that the retard side only pulls the diaphram back from the advance position.
It sits at rest with no advance, so you would not be able to move it by applying a vacuum to the retard side....you would be able to hold pressure though.

RIch

It hasn't run yet, Rich. I'm finishing my first T4 rebuild and was checking the dizzy out when I noticed this. (IMG:style_emoticons/default/poke.gif) No one ever corrects you, Rich. BTW, I appreciate anyones help, right or wrong. TY

When I pulled on the advance side(I refuse to say sucked) the armature would move but it would not hold a vacuum. I can't squeeze the trigger fast enough to produce a measurable reading and it won't hold with my tongue, either. The retard side produces no movement at all. So it sounds as if it might be dead. Next question; is this part still available?

I probably have a couple good ones laying around. It should hold a vacuum for quite some time.

I'll check back with you after I get a pn off the dizzy. Thx

question.
Does the car idle ok when you pull the advance/retard hose and plug it?

You can drive the car without the canister...it just starts the advance faster or pulls the plates back (retard) sooner for emissions.

I am pretty sure (but I am certain I will be corrected if I am wrong) that the retard side only pulls the diaphram back from the advance position.
It sits at rest with no advance, so you would not be able to move it by applying a vacuum to the retard side....you would be able to hold pressure though.

RIch

Wrong. The retard pulls it down from the rest position, the advance pulls it up from the rest position. That way the idle is dropped with the throttle closed (timing retarded), and the timing advanced over time when the throttle is opened (adding to the mechanical advance). Neither side holds "pressure", only vacuum, which isn't measured in PSI, but in inches of mercury.

The Cap'n

So it should move the arm when vacuum is applied to retard side as well, which it does not. Thanks for the tip as to how to ident one. BTW, vacuum can and is measured in inches. FYI, it's about a 2 to 1 ratio. 10psi = 20.36 inches merc. Ever ask yourself why someone would use mercury as opposed to water? Anyway, I have both styles of manometers so if you know an exact value I could use it. TY

I had wrong vac. units in cars already. Be sure to cross reference it. The tech tips 700? lists which ones goes with which distributor. Rich at High Performance house might have a NOS one for you.

And was there any noticeable differences between the different vac canisters. Weren't these referred to as dashpots once upon a time?

[914_teener]

The Cap'n
[/quote]
So it should move the arm when vacuum is applied to retard side as well, which it does not. Thanks for the tip as to how to ident one. BTW, vacuum can and is measured in inches. FYI, it's about a 2 to 1 ratio. 10psi = 20.36 inches merc. Ever ask yourself why someone would use mercury as opposed to water? Anyway, I have both styles of manometers so if you know an exact value I could use it. TY

I believe the values are listed on the Pelican Board for the different dizzes for the Tech Article for timing our cars.

BTW....I am interested in this post as I believe a have a problem with mine as well. I am wondering with some parts becoming difficult to find. Why run it?

My car will never be smoged as it is a 73.

What are the disadvantages if any? No hijack intended.

[orange914]

you would be able to hold pressure though.

RIch

Wrong. Neither side holds "pressure", only vacuum
The Cap'n

Oh Contraire, Neither pressure nor vacuum but "negative pressure"
(IMG:style_emoticons/default/stirthepot.gif)

[tod914]

Made some difference when I put the proper one on. Little smoother under acceleration. Little more power. Nothing to write home about, but worth the swap.

[Cap'n Krusty]

you would be able to hold pressure though.

RIch

Wrong. Neither side holds "pressure", only vacuum
The Cap'n

Oh Contraire, Neither pressure nor vacuum but "negative pressure"
(IMG:style_emoticons/default/stirthepot.gif)

There are multiple terms and situations covered by the term "negative pressure", and "vacuum" is one of them. Rather than using a general term, I chose one that is more specific as well as being more commonly used in the trade. Neener, neener, neener .........

The Cap'n

[SLITS]

Hey Kent .... I have some new negative pressure vacuum canister dashpots. Retard at 7" and Advance at 7".

While physically the same as the 914, they are for the 924/924s.

Stop by and get one. It'll make your car 50 MPH slower than it already is. It will be helpful not to scare yourself.

[Dave_Darling]

The factory manual has diagrams of vacuum versus degrees of advance and retard. I think the numbers from those graphs wound up in the distributor specs in the "914 info" at the top of every page on this website.

--DD

[76-914]

The factory manual has diagrams of vacuum versus degrees of advance and retard. I think the numbers from those graphs wound up in the distributor specs in the "914 info" at the top of every page on this website.

--DD

I'm not finding that here. However, I did find a chart for 2 or 3 different dizzy's on your PP site. Nothing that covered all of them. That being said; I have a question re: how to use the posted info. For instance: Distributor 022-905-205D. Is the idea to start of at somewhere between 1050-1200 rpm then increase the rpm quickly 1500.The dizzy should then advance 14°-17°. And if run up to 2000rpm from the afore mentioned start point it would advance 17°-20°. And at 2900rpm the centrifical advance should be a 22°-27°. Whereas, the vacuum advance should be measured from start point @ 100-130 mm hg then up to 2900 rpm where it should read 200mm hg or 11°-14° Adv. ???????
Secondly, is the advance dial on the timing light used to verify these values?

[Dave_Darling]

Wow, I really thought the charts had made it into the tab at the top of the page. And I couldn't find the info you mentioned on Pelican.

So, here's a chart from Way Back When:

http://web.archive.org/web/20031213100259/...ch/distrib.html

If I recall correctly, I made all of the advance/retard figures into crankshaft degrees when I put the chart together. Also crankshaft RPM, so you can just look at the tach.

The general idea is that you have the base static timing, which is set by rotating the distributor body. Then you have the centrifugal advance, which is added to the static timing depending on engine RPM. Then you have the vacuum advance and retard, which are added to the timing after that.

To verify those figures, you would either have to set up a degree wheel or degree scale of some sort on your fan shroud, or use the advance knob on a timing light equipped with one and check what knob adjustment results in the TDC mark showing up in the timing notch.

So you could check the timing at idle, and at 1500 RPM, and at 3500 RPM, all without the vacuum advance/retard hooked up. Then you could use a vacuum pump to pull a specific level of vacuum on the advance only, and run the engine at 1500 RPM (or 3500 RPM) and see what knob setting shows the TDC mark at the timing notch. The difference between the centrifugal advance and the one with the given level of vacuum on the advance fitting is the amount of vacuum advance. Repeat at different vacuum levels, and repeat all over again with the retard only.

Kind of a pain, if you ask me...

--DD