Vacuum Advance 101

MustangDadDrake

Well-known member
Supporter 2021
Hey Guys,

Looking for some education on vacuum advances.

Some of you know already but I’ll recap... I have 68 Mustang with the non SCV carb and the matching distributor with points. (Auto trans, no ac, no power steering).

I went back and followed Bubbas steps to practice tuning and make sure she is running well. This last time I was messing with the vacuum canister and noticed that when I unplugged the hose and plugged it from the canister there was no change in timing or engine sound. Which makes me think it’s a leak in the hose. I haven’t dug into yet because I wanted to get some more information.

Can anyone explain vacuum advance, mechanical advance, total timing? (Kinda broad)

Also, what should I do that the vacuum advance isn’t working?

Last, I plan to switch to the electronic ignition system, anything I should know in regards to the timing?

Thanks in advance this place is full of knowledge!

PS- I watched this video to get a little bit of knowledge on the canister, wondering if I should adjust it the the Allen wrench as they did.
https://m.youtube.com/watch?v=Qz_cYOe2B20
 
Ok for this discussion pertaining to a 1968 Mustang 200 using the late model point Distribitor
1. First is the Base Ignistion Timing which is set with a timing light with the engine at Idle RPM and the Vacuum Advance line disconnected and plugged.
2. Mechacnical Advance uses weights and springs to control the amount of Advance by the Distribitors RPM.
3. Total Ttiming is a combo of Base Timing and the Mechanical Timing.
4. Vacuum Advance stock it uses ported Vacuum at the carb base to increase the hyway cruse timing to be increased even more. With a carb ported vacuum source there is no vacuum advance at your Idle RPM.

To check the Vacuum Advance Canisters operation you can remove the cap use a vacuum pump or suck on the hose watching if the point plate moves. You can also use a timing light for this test with the engine running. Next hold the vacuum by placing your tongue over the end of the hose If the point plate stays advanced the canister is still good. If it moves back its bad and needs to be replaced. Good luck (y) :nod:
 
x2 what Bubba said, there's no vac advance at idle. Technically, there's no reason to plug it when setting the timing unless you're using manifold vacuum. The port is above the throttle plates and doesn't see any vacuum until you get above idle speeds. Also, most ford vacuum cans are adjustable with a 3/32" (iirc) allen key, you stick it into the hose port. It won't alter the amount of advance but can affect when it comes in.
 
Ok...so no vacuum advance at idle and no vacuum advance at full throttle correct?

The total timing is mechanical + initial? This is confusing to me because where does the vacuum advance come into play, is that not a part of total timing?

If there is no vacuum advance at idle why would I need to plug the vacuum line to set initial timing?

Thanks for the support!
 
Sorry to double post but I went and checked some things per Bubba's suggestion:

Reference the attached photos. I disconnected the blue line from the carb side and sucked. I hear a noise on the diaphragm but nothing moved inside the distributor. Next I started the car with the timing light and I disconnected the blue line from the carb side and put my thumb over it and my tongue :p I did not see a change in the timing.

Just to see what would I happen I disconnected the red line from the intake side and I could hear the air but when I plugged it, there was no change.

Can you guys double check that my hoses are properly connected? Green to Green and Blue to Blue and Red to Red. Also what is the green line and red line called?

Also Chad, not sure if I sucked too hard on the lines and lost brain cells but the links you sent are not working for me.

https://ibb.co/m8cpvxQ
https://ibb.co/ySzMzJL
https://ibb.co/SBVcDJ3


Edited it because I did more tests and found there is an attachment feature.
 

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Plugging it is just a precaution, it's standard operating procedure to insure you are setting the initial timing correctly, w/o variables. The vacuum advance comes into play at high to mid vacuum conditions such as off idle and cruising. It helps economy and responsiveness at part throttle when the mechanical advance isn't yet all in and drops out as engine vacuum drops under high load/wot conditions and keeps you from pinging under those conditions from having to much timing in. Basically it fills in the gap between mechanical timing and initial timing giving you full timing in between idle and wot. Which helps economy and efficiency. The only reason you have no vac advance at idle is b/c of the design of the carb to eliminate it until the throttle plate(s) crack open further than idle. They do this so that the engine will crank properly when you are starting. If you had 12 deg of initial and the vacuum is throwing in a sporadic 5-10 deg more as it cranks it can be hard on the starter motor and cause the engine to crank more slowly. Having this vac advance in at idle can be useful if you want or need to lower your initial timing but want or need to have more timing in at idle. In that case you would use manifold vacuum instead of the ported vac source on the carb. This would give you higher idle timing and allow you to lower the idle speed screw to get to the same idle speed. For example this can cure a run-on condition on an engine that has low initial timing(but doesn't want more) by allowing you to close the throttle blades more at idle(idle speed screw) than when using the ported vacuum source on that engine.
 
MustangDadDrake":2271t3ip said:
Also Chad, not sure if I sucked too hard on the lines and lost brain cells but the links you sent are not working for me.
Seth nails it in #7 post.
The links wrk & don't wrk...
Not sure what's goin on w/them.
Have linked here B4, will stop now...too bad. were good.

Many/most Qs can B answ in the above "tech archive". It duz
hasea vac advance sectin...
 
Now you should have a working knowledge of advances.
See my ad in the small six for sale section, when you are ready to step up to electronic.
viewtopic.php?f=86&t=79961
I offer the DS11 with the mechanical advance curve custom done for your engine combo. The vacuum advance will also be custom done for your engine as well. Bill
 
Howdy DadDrake and all:

Are you aware that you are working with a vacuum advance/retard system on your distributor? IT was a short term crutch by FoMoCo used in 1968 and 1969. Beginning in 1970 they switched to a Carter YF carb. In 1974 they introduced the first generation of the Dura Spark, pointless distributor.

What worked for me, back in the early days of figuring this out was to remove the vacuum retard hose and plug the hose. Leave the nipple nearest to the distributor cap open. The outside nipple is now working as a vacuum advance with a ported vacuum source from the carb. Make sure the vacuum canister is working. The vacuum advance with this carb and distributor is primarily for MPG and reducing emissions.

Also know that the '68 Autolite 1100 is down on cfm as compared to '66 Autolites. 150 cfm to 185 cfm.

It is good to know these things. Keep it coming.

adios, David
 
MustangDadDrake:1523i498 said:
Sorry to double post but I went and checked some things per Bubba's suggestion:

Reference the attached photos. I disconnected the blue line from the carb side and sucked. I hear a noise on the diaphragm but nothing moved inside the distributor. Next I started the car with the timing light and I disconnected the blue line from the carb side and put my thumb over it and my tongue :p I did not see a change in the timing.

Just to see what would I happen I disconnected the red line from the intake side and I could hear the air but when I plugged it, there was no change.

Can you guys double check that my hoses are properly connected? Green to Green and Blue to Blue and Red to Red. Also what is the green line and red line called?

Also Chad, not sure if I sucked too hard on the lines and lost brain cells but the links you sent are not working for me.

https://ibb.co/m8cpvxQ
https://ibb.co/ySzMzJL
https://ibb.co/SBVcDJ3


Edited it because I did more tests and found there is an attachment feature.

Sounds like your Vacuum Advance is defective and needs to be replaced. You have a two port vacuum advance, the second or rear port is to retard the timing. This type can be replaced with a regular one port vacuum advance for better economy. Good luck (y) :nod:
 
Ok I think I am figuring this all out.

David, if my vacuum advance is currently working...I would unplug the hose, one closest to the distributor cap, and leave it open. Then I would cap the hose, can I just remove the hose as well and cap where it is coming from? See pictures 1 and 2.

Bubba, if my vacuum advance is not working, which I do not believe it is but I want to try your tests one more time and take it off the car to check it out. I would get this vacuum advance with only 1 port. It would essentially do the same as what David is suggesting. The listing on NPD says it is not for my year car, which makes sense. But would this vacuum advance fit my distributor or how could I figure it out.
https://www.npdlink.com/product/diaphra ... &year=1968

Econoline, thank your for your post I had to read it a couple of times but it started making sense to me now.
 

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MustangDadDrake":2xwgi52s said:
Ok I think I am figuring this all out.

David, if my vacuum advance is currently working...I would unplug the hose, one closest to the distributor cap, and leave it open. Then I would cap the hose, can I just remove the hose as well and cap where it is coming from? See pictures 1 and 2.

Bubba, if my vacuum advance is not working, which I do not believe it is but I want to try your tests one more time and take it off the car to check it out. I would get this vacuum advance with only 1 port. It would essentially do the same as what David is suggesting. The listing on NPD says it is not for my year car, which makes sense. But would this vacuum advance fit my distributor or how could I figure it out.
https://www.npdlink.com/product/diaphra ... &year=1968

Econoline, thank your for your post I had to read it a couple of times but it started making sense to me now.

For swaping to single vacuum advance canister you can't go by the same year application. Those in your above listing are only showing as V8 applications. You can look for one for a 1970 or 71 Ford Maverick 200 six it well go right onto your Distribitor mourning just as the stock old one. As far as the extra vacuum lines you could disconnect it all the way back at the carb port (the Red lines fitting) and cap it off then remove all the other Red and Green hoses if you want, then retry the test to see if the vacuum canister is working or not. Good luck (y) :nod:
 
"...you can't go by the same year application..."
However, U can - w/correct application (200 v 250) motor for that yr for your PCV valve. I say this as I saw a pic of the rear valve cover (I think it was your engine) w/a hose connected from back valve cover w/o a pcv in it.
 
Alright guys think I am all dialed in on the vacuum advance.

Now that I understood it better I went back and tested the vacuum advance and was able to get the arm to move.

I also removed the hoses for the vacuum retard and left it uncapped. I then capped the other ports.

I took her out for a spin and it sounded nice. I didn't go up any hills but I did drive it right in that mid range, with no pinging or other odd noises.

In the future if I wanted to get that better fuel economy, these are the vacuum advances I would want to get from a 70s Maverick, right?
https://www.ebay.com/itm/Distributor-Va ... 1438.l2649

https://www.ebay.com/itm/Niehoff-FF-318 ... 1438.l2649

Chad, thanks for looking out but I think I have the proper PCV hook up.
 

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As long as the current vacuum advance is working and the retard is disconnected that is all you will need for good economy. So I wouldn't change it unless it quits working. Good luck (y) :nod:
 
In a non-Load-O-Matic distributor there are 3 advance settings/components. Static timing is where one sets the distributor using the timing mark on the balancer/crank pully. Mechanical timing is determined by swing-weights and springs. As RPM increases the swing-weights move outward...advancing timing. The springs limit how fast the weights can move, so different springs speed up or slow down advance. Vacuum is a way to help economy in stop-and-go driving. It aids by advancing timing as one releases the throttle...if hooked to manifold vacuum. Ported vacuum increases vacuum as speed increases but decreases as speed decreases. If one has 12 degrees BTDC to start, and 20 degrees with swing-weights you get 32 degrees total before vacuum is counted. At highway speeds, after the throttle is stabilized and engine vacuum increases, vacuum adds a few degrees advance. So, 12 degrees, plus 20 degrees weights, plus 2 to 4 degrees vacuum gives 36 degrees total. A Load-O-Matic distributor all advance after static is set, is controlled by ported vacuum and the 1100 carb advance control valve on the side of the 1100. I prefer manifold vacuum on my engine the way it is set up.
 
12 degrees initial is not productive. However you tailor your initial to the engine combo. Then you set the total centrifugal advance & calibrate the vacuum advance.
 
So, wrk w/the initial? ck in with the mechanical (weights/springs)...leave vac till last...?
(of course w/carb tuning as well)
 
Happy Sunday!

Did another run through on the tuning now that I better understood the vacuum and mechanical advance.

Ran into something similar to what happened to the guys on the video I posted in post # 1.

Engine was warm and I confirmed 12 degrees initial timing. I put it up to 2500RPM with the vacuum advance connected and I was at 60 degrees. Then I disconnected the vacuum advance and plugged the hose, put it back up to 2500RPM and was at 32 degrees timing.

I tried to get in the vacuum advance canister with an 1/8" hex to adjust it but I was having trouble. So I took it off the distributor to get a closer look. The advance port was a little dented up but I was able to access it with the hex but it still wasn't sliding into the hex head. I heard a rattling noise in the canister and when I shook it (not sure why that was my natural instinct) then black stuff started coming out of it see the the 3rd picture. So I ordered one of the ones in post # 15.

Let me know if I am off base here but my vacuum advance should not be giving me an additional 28 degrees of timing at 2500rpm and since I couldn't adjust it I bought a new one. Make sense :unsure::
 

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