Quadrajet Carburetor Mixture Screws

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c3po

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I was looking on a website and they said the best way to adjust the idle mixture screws was to get a vacuum gauge.

They said to adjust the mixture screws by using the ported vacuum from the intake manifold.

A guy who used to be a mechanic told me to use the ported vacuum from the carburetor.

Has anyone ever adjusted a Quadrajet with a vacuum gauge, I guess I would need to get it to 18 in. on the vacuum gauge.

I would guess the car would need to be warmed up first.

Any thoughts would be appreciated.
 
I got a better idea. Google TBI swap. It's not a terribly difficult job and the results would be worth it.
 
Clayton, thanks for that article.

What is the difference between manifold vacuum and ported.

There is a tee on my intake manifold that has a vacuum cap or two at the back of the intake manifold.

So basically I am taking the vacuum gauge and I am putting it on manifold vacuum.

I do have a vacuum gauge, I guess that I will have to read the face of the gauge too see where I am at. I will look at the gauge when I go out too the garage tommorow morning.
 
Better to be a slight amount rich than too lean.

If I remember right ported vacuum is only available at of idle.
Manifold will have constant vacuum, ported will not at idle. Unless your carb is way off then ported could be working though it should not be.
 
Ported vacuum isn't available at idle.

That's so the vacuum advance is inoperable at idle.

Better to be a little lean at idle, a tad leaner than best RPM/vaccum.
 
Hook up the vacuum gauge to manifold vacuum. If your manifold is a two plane design, make sure the vacuum port sees both planes of the manifold. Quadrajets will usually have a port for manifold vacuum, IIRC.

I used to just use an analog tachometer for mixture adjustments. Slowly screw in one side until you see a drop in RPM. Back off the screw until the RPM peaks. Repeat for the other side.
 
The highest vacuum reading will net you a bit too rich.
Go for a lean drop of 25 RPMs, maybe 50 RPMs.
Everything else must be perfect before you set the carb.
 
Ported vacuum comes from a hole drilled above the carb's throttle valve that is "swept" when the throttle valve is opened. As the hole is swept, a vacuum signal is produce that diminishes as the valve opens further. This type of port was used primarily to tailor the vacuum advance curve. It is not of use for idle vacuum; look for manifold vacuum. Vacuum at idle is affected by the camshaft's overlap. I doubt you'll see 18 inches at idle, 15 to 16 is more likely. Adjust the idle screws slowly, with the engine warmed up, and stop at the highest vacuum reading on each.
 
I went ahead and contacted Sean Murphy, at Sean Murphy Induction, the place where I bought the carburetor.

He stated that I needed to use manifold vacuum. I have a spot on the back of the intake manifold where I can take a cap off and get manifold vacuum.

I will adjust the idle screws slowly until I achieve the highest vacuum reading on each screw.

mechtech2 has said that the highest manifold vacuum may net the carb a bit too rich. Does this mean that I may have to turn each screw a little bit lean.

flanso has said that 18 inches at idle may not be realistic, 15 or 16 is more likely, if I lean it out would 14 inches be okay.

I plan on doing this tommorow, and I will let you guys know how everything goes.
 
Originally Posted By: paulo57509
Hook up the vacuum gauge to manifold vacuum. If your manifold is a two plane design, make sure the vacuum port sees both planes of the manifold. Quadrajets will usually have a port for manifold vacuum, IIRC.

I used to just use an analog tachometer for mixture adjustments. Slowly screw in one side until you see a drop in RPM. Back off the screw until the RPM peaks. Repeat for the other side.


Exactly, I never had a problem doing it that way. The vacuum method doesn't always work well with an engine that has very high miles or is tired.
 
Ah the wonder of the Rochester Quadrajet, the mini primaries for economy and throttle response, and those massive secondaries for power. Only trouble was everybody knew how to tune a Holley but nobody here had the expertise to tune a QJ. Lack of tuning parts was a problem, but basically just a lack of knowledge. The most under rated carb ever IMO.
 
Next thing c3po will want to know is what size horseshoes to use on the "Ol' Nag"....
LOL.gif
 
It's been a while, but from what I remember, I never used a vac gauge to adjust my idle mixture. Open the screws 1 1/2 turns from closed, then back them off until the highest possible idle, then turn clockwise 1/4 to 1/2 turn (until you hear the idle going down a little). It's not precise, but it doesn't need to be.
 
Hopefully I can get out in the garage either tonite or tommorow, I do not think I will have a problem with the vacuum gauge.

I remember when I bought it in the summer time and checked the vacuum readings from the manifold and carburetor and it seemed like the car had good vacuum.

As far as wear on this engine, it does not use or consume oil, no oil leaks, no oil burning or puffs of smoke when starting it.

I just need to fine tune this carburetor.
 
Originally Posted By: c3po


mechtech2 has said that the highest manifold vacuum may net the carb a bit too rich. Does this mean that I may have to turn each screw a little bit lean.


Yes

Originally Posted By: c3po


flanso has said that 18 inches at idle may not be realistic, 15 or 16 is more likely, if I lean it out would 14 inches be okay.



You aren't shooting for a specific number. You back off just slightly from the highest, whatever that is.
 
I got out there today and I hooked up the vacuum gauge and it was reading between 19 inches and 20 inches, and I backed off to about 19 inches.

I decided to check my intake manifold bolts, and a couple of them were loose, so I tightened them. The car ran pretty good, it was 40 degrees outside.

I think Demarpaint was right in that carburetors like 55 degrees or better outside, it ran great when it was 72 degrees last weekend.

I still think there may be a slight vacuum leak, or maybe I need to change my fuel filter, when the car is totally warmed up and I put it in reverse, its like the engine is shaking a little bit.

If I had between 19 and 20 inches of vacuum what does this say about the condition of the motor, or maybe this means nothing.

The only other thought is that I could spray carb cleaner around the base gasket of the carb where it meets the intake manifold, or spray it around the vacuum caps and the vacuum lines.
 
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