Saved my vacuum advance dissy...and a few bucks.

[duffontap]

I discovered the other day that my vacuum advance wasn't working anymore so I started looking for replacement parts/units. After core deposit, Napa's remanufactured units are about $140, which seems reasonable enough. I only need the vac assembly, but the only place I found one was on SOR--used for $75. Did a little searching and found this:

https://forum.ih8mud.com/40-55-series-tech/281609-vacuum-advance-repair.html

VERY helpful. I tore into mine, figuring that a core is a core and I didn't have much to lose. I found that my vac diaphragm was intact, but the rubbery sealant was pealing off the cloth. A couple minutes with a can of carb cleaner left my diaphragm looking like this:

And after a coat of spray Plasti Dip it held vacuum and after four coats it was flexible, held vacuum, and looked like this:

Re-crimped the assembly, returned it to the dissy and reinstalled the distributor. My truck has never run better. I hope this might help someone else who's starting to notice their vacuum advance isn't what it used to be.

Now I have one question about hooking it up to the carb. I had a feeling that I might have hooked it up to the wrong vac port so I hooked my vac tester up to the various engine side ports and watched the dial from idle on up. The one closer to the headlights has zero vac at idle and slowly generates vacuum at higher engine speeds up to about 5", and the port closer to the taillights has no vacuum at idle (both ported obviously), and quickly generates idle up to about 15" in the same rpm range. Which one do I use? It's a '78 Aisan. Cheers, Duff.

 

[duffontap]

Any help on the question ^^^^^ would be appreciated. I don't want to drive it until I get a definitive answer because I'm worried that it might really matter. I've spent so much time searching this site and coming up with apparently contradictory answers that I wanted to just ask.

The question again is, I seem to have two ported vacuum ports at the base of my '78 Aisan carb. Which one should I use because they have very different vacuum readings when I rev the engine.

Thanks!

 

[Pighead]

I'm sure someone will chime in with the correct answer any minute now. In the meantime, maybe try driving it on one and then the other vacuum port and see which you like better.

 

[duffontap]

I'm sure someone will chime in with the correct answer any minute now. In the meantime, maybe try driving it on one and then the other vacuum port and see which you like better.

Thanks Pighead. I drove it both ways yesterday--just for a few minutes--and then began to worry that the port with the lower vac reading wouldn't actuate the advance, and the port with the higher vac reading would damage my newly repaired diaphragm. Paranoid, I know.

Brief Rant:
This is when I just hate being desmogged. When I have questions like this I search the FAQ and tech forum (sometimes for hours) without finding a single definitive answer. I blame the fact that nothing is 'standard' once you rip out the factory vacuum system. I'm honestly thinking about resmogging so I can find answers in a manual! Of course, killing hours searching the FAQ is a reward in itself. Ha, ha.

 

[duffontap]

Well, for posterity's sake: I finally settled on hooking it up to the vac port with higher readings. It's running better than it ever has while I've owned it. I'm currently getting crazy good gas mileage.

 

[Pork-n-Bean]

Awesome fix - great ingenuity!

 

[FJsnoozer]

i have no idea if this helps, but since no one has answered yet.....

if you notice pinging on one... dont run it

You're probably thinking, "Sure there is no manifold vacuum at WOT but aren't I supposed to use ported vacuum for the vacuum advance." Hold onto your hat, THEY ARE THE SAME THING! Except ported is shut off at idle. There are a lot of misconceptions when it comes to the ported vacuum source. After hearing 20 different theories I decided to hook up two vacuum gauges, one to manifold and one to ported, then drive my car and watch it. I found out they are the same, except the ported is shut off when the throttle is closed. Even then I had a hard time convincing guys so I hooked up a couple MAP sensors and a throttle position sensor to a data logger and recorded them while driving then dumped it into a spreadsheet and made a chart. As you can see, there is a direct relationship between throttle position and vacuum. When the throttle is closed vacuum is high, when the throttle is open vacuum is low, and ported vacuum is the same as manifold except when the throttle is closed. So which one do you want to hook it to? I prefer manifold vacuum. This pulls in more timing at idle which is good since there is virtually no load. Your motor will idle smoother and cooler with the extra timing. One night I was at the drags and my car was running hot in the staging lanes, I swapped the vacuum advance from ported to manifold then it would idle all night at 175°. Believe it or not the purpose of ported vacuum is to raise the temperature at idle, to lower NOx emissions. If you're like most hotrodders that is of no concern to you. If you have a big cam with a choppy idle then a vacuum advance hooked to manifold vacuum can really help. It will idle smoother and requires less throttle to maintain speed. Often a big cam requires you to open the throttle so far that the curb idle adjustment needles won't work. Hooking the vacuum advance to manifold vacuum will allow you to close the throttle some which may be enough for the idle mixture screws to work. Someone told me he noticed less dynamic braking with the vacuum advance hooked to manifold. I didn't notice it on my car but it makes sense. If the motor is running more efficiently with the added advance it will make a less effective brake. So which should you use? Try both and see which you like best.

Once you have the mechanical advance setup to give you the most power, and no pinging, at WOT then you should setup the vacuum advance. A stock vacuum advance will pull in 20° or more. If your car is pinging or running rough after hooking up your vacuum advance then you need to turn it down. Most vacuum canisters are adjusted by sticking an allen wrench in the vacuum tube. Turning the wrench counterclockwise will reduce the timing. Just turn it down a bit at a time until the problem goes away. I had to turn my vacuum advance down until it only pulled in 5°.

Distributor advance

 

[numby]

The vacuum ports (for ported vacuum) are located on the valve cover side of a '78 carb. You'll have to remove the air cleaner assembly for a closer look...

If there are three ports then use the one closest to the firewall for vacuum advance.

 

[duffontap]

If there are three ports then use the one closest to the firewall for vacuum advance.

That's the one with the highest vacuum reading after all.

Thanks guys. This is interesting stuff to me but I have been having a little trouble finding definitive answers to some of these questions, so your help is much appreciated.

 

[JohnnyC]

interesting...I should check mine

thanks for posting

 

[ducktapeguy]

That's a great idea, I repaired mine with a spare secondary gasket that I glued to the original, but I like your idea better. If it ever leaks again, I'll probably do the same thing. I don't know if it was the vacuum advance, the new carb, but once everything was fixed it felt like I had a new engine in there.

 

[duffontap]

That's a great idea, I repaired mine with a spare secondary gasket that I glued to the original, but I like your idea better. If it ever leaks again, I'll probably do the same thing. I don't know if it was the vacuum advance, the new carb, but once everything was fixed it felt like I had a new engine in there.

I hope you saw that I linked your diaphragm repair thread in my first post. It was really helpful!

I don't know how long it's been since I have had vacuum advance but my mileage did go down a few months ago. I went on a road trip yesterday and put 650 miles on the OD at highway speed. My first tank was 17.01 mpgs--my best mileage ever!

 

[numby]

If there are three ports then use the one closest to the firewall for vacuum advance.

I should have added that if your '78 Aisin carb only has two ports then it's either a Cal spec model or a high altitude model. California models do NOT technically have ported vacuum. One of the ports IS for the EGR system though. This acts like ported vacuum but comes in later as you open the throttle. Could work in a pinch I guess. I'd check with Mark A. or Jim C. to be sure.

 

[ducktapeguy]

I hope you saw that I linked your diaphragm repair thread in my first post. It was really helpful!

Ha! I guess I missed that. Glad it helped, I thought I was the only person too cheap to spend the money for a piece of rubber from SOR.

I don't know how long it's been since I have had vacuum advance but my mileage did go down a few months ago. I went on a road trip yesterday and put 650 miles on the OD at highway speed. My first tank was 17.01 mpgs--my best mileage ever!

My best was about 16mpg before the swapping the carb, drilling for ported vac, and fixing the advance, haven't taken it on a long enough trip to accurately calculate it since. My mileage around town stayed about the same, but the grin on my face definitely increased due to the added power .

 

[65swb45]

Interesting.

I am curious to know how many cuts you put in the seam of the canister in order to fold the lip back? Any pics?

Plasti-dip is great stuff. I used it to repair and remold the bugcatcher weatherstrips in the lwb45. Brush on, let dry, razor blade to shape.

Best

Mark A.

 

[duffontap]

Interesting.

I am curious to know how many cuts you put in the seam of the canister in order to fold the lip back? Any pics?

Plasti-dip is great stuff. I used it to repair and remold the bugcatcher weatherstrips in the lwb45. Brush on, let dry, razor blade to shape.

Best

Mark A.

Hey Mark,

Actually, I didn't have to make any cuts. I used wide open Channel Locks to work my way around, crimping the outsides toward the center, and then finished with Vice Grips. It isn't pretty, but the edge did fold down fairly evenly and it ended up nice and tight.

I actually bought that can of Plasti-Dip to restore some window seals--I'm amazed at how tough that stuff is. Cheers, JD

 

[tls3601]

Awesome, I have a '78 as well and the OEM rebuilt by Jim C is going go back on soon. Great fix

 

[NeverGiveUpYota]

Hey Mark,

Actually, I didn't have to make any cuts. I used wide open Channel Locks to work my way around, crimping the outsides toward the center, and then finished with Vice Grips. It isn't pretty, but the edge did fold down fairly evenly and it ended up nice and tight.

I actually bought that can of Plasti-Dip to restore some window seals--I'm amazed at how tough that stuff is. Cheers, JD

Hey Duffontap... Hows' this rebuild with the vac adv going for you now? Are you still driving the truck? I recently pulled apart a vac adv for my 60... Thread posted yesterday and a few members linked this one about the plasti-dip. I am curious how its holding up. Thanks for any input you can provide.
Felicity