I have always used the manifold vacuum. I think this gives you a truer idea of what your right foot is doing to the dizzy. Ported vacuum will give a true reading of what the cylinder sees however. But in my mind, you want the dizzy to respond to what the foot does because that is what you want the engine to do. Also, there is a momentary hesitation in vacuum and air flow to the cylinder when you crack the throttle open. To compensate for that, you have the accelerator pump. My opinion is that you need to factor that out of the vacuum advance.

Make sure you are not sucking any unwanted air into the system. Also loosen the screws on the butterflies and reseat them with the idle screw backed off and see if that helps your mixture problems.

 

Man, I think you just opened up a can of worms! I use ported vacuum, and adjust the vacuum operated advance on the dizzy to operate to my liking.

It is adjustable with and 1/8 inch wrench stuck in the tube where the vacuum hose goes.

 

This topic does open a can of worms. On my 360, i tried ported and manifold vacuum and manifold vacuum gets me a smoother,cooler idle, 200rpm,s and 1" of vacuum. I run a Holley carb,and I hooked a vacuum guage to both ported ,and manifold vacuum sources, and drove around .The vacuum readings were identical, except for idling, where ported was zero,and manifold was at 19". As for vacuum advance, they,re the same except when idling where ported gives you only initial timing, and manifold gives you initial+ vacuum. As soon as you touch the throttle, the differences are gone. I run manifold vacuum,and My motor can idle in stop and go traffic in 96 degree heat,and my water temp. guage bounces between 175 and 180 with a 180 thermostat. Throttle response is great. You do have to know what your initial, total timing is,and how much vacuum advance you are getting from your distributor to be safe.

 

Depends alot on your ignition. I have a DUI distributor (HEI type) on my 4.2, and DUI says that their product seems to do better on manifold vac. I tried it and they were right. I wouldn't operate a stock dizzy on it though.

 

....... I tried it and they were right. I wouldn't operate a stock dizzy on it though.

If you purchased your CJ new then you would have, all of the built for U.S. '83-'86 in-line six as well as the '84-'86 four banger used manifold for vacuum advance.

As for that JeepHammer thread while it is a very good primer it isn't written without flaw.

 

http://www.jeepforum.com/forum/f12/manifold-vs-ported-vacuum-994651/

The first post says all that needs to be said...

 

I have read this article many times before. However, if you read JeepHammer's post on tuning your dizzy (http://www.jeepforum.com/forum/f8/working-your-distributor-how-tune-973904/) you will see that he recommends to run off of ported vacuum. Perhaps that is because it is a modified stock setup that he is talking about and not a all-in-one HEI dizzy.

 

Here's another good article.

http://www.corvette-restoration.com/resources/technical_papers/Timing101.pdf

As to the ported signal being the same as the manifold signal once the throttle is cracked: maybe sometimes but usually no. An aftermarket carb usually seems to be the closest and the "smoggier" carbs seem to be quite different.

My Weber is pretty close. When you give it throttle, the ported signal goes up smoothly while the manifold signal drops smoothly and they're about equal from nearly half-throttle on up. My old BBD was quite different. The ported signal was quite lazy coming in and then it would jump to and remain at only about 7 to 8 inches of signal for awhile and then jump up again to 15. It wouldn't start coming down from 15 until you were pretty deep into the throttle. Oddly enough, that length of time where it stayed around 7 or 8 inches and delivered hardly any advance at all was enough rpm to complete the high-speed sniffer test, then it would jump up :idea:

I switched the BBD and Motorcraft over to manifold vacuum but I needed to make some adjustments to the vacuum canister to make it right. Just moving the hose from one port to the other can leave you pingin' or sluggish or some combination of the two so some calibration may be in order.

Shawn

 

Shawn,
Which way did you have to adjust your vacuum canister? Did you make the spring rate stronger, or weaker.(did your turn the allen wrench clockwise or ccw?)

 

Shawn, I'm glad you've posted your findings...mine were somewhat similar, while the road test were with a 32/36 DGV mounted I found the ported signal at cruise a bit stronger and much sooner than yours. Just as you said the ported signal was considerably stronger than the manifold at the higher rpm.

Your point about the differences in that signal between carbs design should be taken as gospel...IMO the same goes for for engine condition and the carb mixture.

Each user should do their own test and make adjustment as necessary.

And I thank you for suggesting that I give manifold vacuum a whirl.

 

Shawn, I'm glad you've posted your findings...mine were somewhat similar, while the road test were with a 32/36 DGV mounted I found the ported signal at cruise a bit stronger and much sooner than yours. Just as you said the ported signal was considerably stronger than the manifold at the higher rpm.

Your point about the differences in that signal between carbs design should be taken as gospel...IMO the same goes for for engine condition and the carb mixture.

Each user should do their own test and make adjustment as necessary.

And I thank you for suggesting that I give manifold vacuum a whirl.

You're quite welcome, sir. I'm glad it worked out :thumbsup:

Shawn,
Which way did you have to adjust your vacuum canister? Did you make the spring rate stronger, or weaker.(did your turn the allen wrench clockwise or ccw?)

Counter clockwise increases the vacuum signal needed to advance it. Just so I knew where the hell I started from and ended up, I adjusted it just like you would a mixture screw on a carb. I counted my way in (how many turns clockwise) from its original position to its seat so I could start over from scratch if I needed to. Then I started backing it out in 1 turn increments (from its original position, not seated) and road testing until I got all of the ping out on a long inclined stretch of road. I think I ended up around 7 turns out or so with the Cali-smog equiped BBD and Motorcraft dizzy on my '83.

Before I started with the canister adjustments, I made sure that it didn't ping on that stretch of road without the vacuum advance hooked up at all so I knew I was adjusting what I needed to be.

Hopefully that makes sense and helps.

Shawn

 

It's been a year and half since I did it so I may have it ***-backwards :dunno:. I can tell you for sure that it pinged in its OEM position and I had to back it out to get it set right.


Shawn

 

CCW increases the amount of vacuum necessary to activate, or in other words delays advance, it also increases the amount of advance available should the signal be strong enough. CW adjustment lowers the amount of vacuum as well as lowers the total amount of advance.

While the '83-'86 advance mechanisms were identical most every year previous, back to the '79 were changing year to year.

 

Shawn, thats interesting stuff about tuning the vacuum advance for either ported or manifold. My adjustable vacuum advance pulls in about 12 degrees ,but I don,t know how quick. I have a stock compression 360 with a performer-plus cam. With 12 degrees initial, 20 more from the HEI, all in at 2200 rpm,s, this motor will not ping ,even down to 1200 rpm. Maybe it could use more vacuum advance?

 

The short answer to your question is, as much timing as you can run without spark knocking or causing it to be hard to crank when hot.

You have 32* now....have you tried 34 or more? I'd turn it up until it knocked, then back it off a couple of degrees.

You could also try some lighter advance springs, which will make the advance come more quickly. You may well already be at the point of diminishing returns, but it certainly won't hurt to fine-tune a bit more.