# Quadrajet Carburetor Adjusting Screws Do Nothing - Any Ideas?



## ColoradoMike (Mar 29, 2020)

Almost done with restoring a 1969 GTO and having a frustrating problem. The car has a completely rebuilt 400 and except for some internal performance parts, is basically stock. I did use an internal ignition module to eliminate the points.

I had purchased a late 1960's rebuilt Quadrajet carb a long time ago and it sat in the box for years until installed recently. This carb would not let the engine idle as it would "surge" from 700 to 1100 RPMs and the motor would eventually quit, then it would be very hard to start. This makes the car undrivable as it could stall at an intersection and then not readily start.

Seeing that getting this carb rebuilt would take months as all the shops are busy, I bought a rebuilt Quadrajet and had it installed. Guess what? The adjusting screws on the front of the carb also do nothing as well! Seems like the carb may not be the issue.

The new carb does idle better, but still, the adjusting screws seem to do nothing. 

We've carefully checked for vacuum leaks and the car does have hidden headlights, so there are a lot of vacuum connections, but even disconnecting them and plugging the ports seems to make no difference. 

Any ideas or thoughts? I likely didn't include some needed info, so let me know what I can answer. I'm going to chance driving the car with this new carb on it to see if putting some miles on it may help, but I can't seem to find any logic for why the adjusting screws don't seem to do anything. 

Below is a pic of the orignal carb I had gotten if that helps.


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## armyadarkness (Dec 7, 2020)

ColoradoMike said:


> Seeing that getting this carb rebuilt would take months as all the shops are busy, I bought a rebuilt Quadrajet and had it installed. Guess what? The adjusting screws on the front of the carb also do nothing as well! Seems like the carb may not be the issue.


Welcome to the club. Happened to me as well. Some rebuilt Qjets are worn out hunks of junk.


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## armyadarkness (Dec 7, 2020)

There may be other issues... but Im just stating, definitely dont assume that a rebuilt Qjet is good.


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## armyadarkness (Dec 7, 2020)

No need to guess... what is your vacuum? If you dont already have a gauge, grab one from the store for $20.

If you have 15hg or better, then it's unlikely that that's your issue.


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## Jim K (Nov 17, 2020)

X2 to Army.
Seems as if there are few companies that understand Rochester carbs today enough to just be able to buy one and slap it on with the knowledge that it will work right, right out of the box.
I'm no Rochester expert but I've been into and through several through the years. Some members on here have a pretty extensive knowledge of them and hopefully they will chime in.
If your idle mixture screws don't seem to be doing anything, first thing to come to mind is it might be because your idle speed screw has pushed the throttle plate to far open and taken the mixture adjustment screws out of the picture. Best thing I can tell you is to get hold of of a good Rochester manual, or a rebuild kit guide and go through all the required adjustments and measurements. Don't leave anything to chance. A vacuum gauge, as Army mentioned, is an almost vital tool when making mixture adjustments. As far as the one you had sitting in a box for years, it will definitely need torn down and all the measurements checked and internal gaskets replaced, but it's probably still a good carb.
Found this old AC Delco manual online in PDF format. Might give you some help.

https://www.generationhighoutput.com/Quadrajet_Service_1981.pdf


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## armyadarkness (Dec 7, 2020)

On my Vette, with a brand new "quality" Qjet, I could close both idle mixture screws and the car still idled at 1000 rpms... with the idle rpm screw completely out!

I threw and Edlebrock on and it's a dream!

On my 67, the car died at stops and it was because the rocker arms were to tight, causing the valves to hang open.


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## Duff (Jan 12, 2020)

Good advice from ARMY, Edelbrock AVS2 and an Edelbrock performer intake.


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## geeteeohguy (Feb 2, 2008)

What Jim K said. Suspect: float level too high, vacuum leaks, base idle speed too high taking idle jets out of the picture.


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## AZTempest (Jun 11, 2019)

Vacuum leak and base idle speed too high would be my guess to start with. 

One time on my Tempest I could not get the passenger side mixture screw to do anything on a Qjet. Drivers side would adjust fine. We tried everything without success. Then one weekend my son and I were installing headers. When we pulled off the passenger side exhaust manifold, we found the heat controlled valve inside the manifold rusted closed. Funny thing is we had blocked off the intake exhaust crossover so we have no idea where the exhaust gases were going to on the passenger side. Once we had the headers installed, the carb would adjust again as needed.


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## LATECH (Jul 18, 2018)

I have built a BUTTload of Q jets. That includes a trailer load of the electronic ones. I have had only 2-3 over the years that had some internal problem that could not be fixed.
One was cracked and couldnt see it with it installed, but it was there. 2 others had air horns that were so warped I shipped them to the recycle bin

Check to be sure the base gasket isnt leaking air , many times the wrong one is used. 
Maybe the Idle channel restrictors are missing ...LOL it would run pig rich, and require a LOT of extra air, which means you would need to crank open the base idle screw, which would also.. wait for it ............disable any effect that the idle screws would have
Check for vacuum leaks.Try blocking the PCV valve to see if that changes anything
Maybe the secondaries are hanging open part way, or the primary throttle plates arent allowing the throttle plate to close all the way due to not being in the bore correctly.
My guess is base gasket causing vacuum leak. 
Look it over . Spray some brake cleaner around the carb base while it is running.If the engine picks up a bunch of speed you found your vacuum leak.
I wouldnt give up on the Q jet. Seems the first thing that people do is jump to the conclusion the carburetor is at fault.It is usually a symptom, not as often the fault


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## lust4speed (Jul 5, 2019)

If the vacuum leak were external the carb adjusting screws would still work, just not all that well. I'd say it is a vacuum leak but with the Quadrajet, and could be a warped top, crack somewhere in the interior passages, or something as simple as someone assembling the carb with the wrong gaskets. There are a lot of different bowl and air horn gaskets and picking the wrong gasket will allow leaks that will kill the effect of the mixture screws.

I once left a PCV valve disconnected from the hose and still managed to drop the idle down and adjust the idle mixture screws. About the time I realized the idle screws were about 10 turns out I knew I screwed up somewhere and the 3/8" vacuum leak was soon discovered. The point being a decent carburetor will have that much adjustment.


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## AZTempest (Jun 11, 2019)

Another vacuum leak I have heard of that's not obvious was the intake was hitting the valley pan not allowing the intake gasket a good seal. Worth shooting some carb cleaner around while running to check.


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## Bopman (7 mo ago)

AZTempest said:


> Another vacuum leak I have heard of that's not obvious was the intake was hitting the valley pan not allowing the intake gasket a good seal. Worth shooting some carb cleaner around while running to check.


Another is throttle plate rods wore and allows air to be sucked in around them. 🤔🤔


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## Bopman (7 mo ago)

Bopman said:


> Another is throttle plate rods wore and allows air to be sucked in around them. 🤔🤔


Or shafts....which ever you want to call the....I guess shaft may be correct 🤷‍♂️


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## Machinest-guy (Jul 19, 2019)

Might want to try taking one adjusting screw out and blowing come Chemtool into the adjuster hole. Engine off. WOT. See if the spray comes out into the venturi area. Then do other side. 

Engine on. screw out. Engine should not idle. But spray more chemtool in and you should be able to simulate an idle mixture with a steady thumb on the can valve. If possible to do that then carb us plugged up internally. 

Another test. use a pencil with eraser tip to block air bleed from the choke tower. It should change engine speed and mixture. 
The forgoing came from GM carb school years ago on the Qjet and a bunch of other Rochester products.

Another test. shut fuel off to carb. Zero inlet fuel and nothing in carb to confuse you. Take a propane torch and waft propane into carb choke tower. You should be able to get car to run and idle. Put rubber hose on the torch snout and just stuff other end down the venture area. Choke blade and cloths pins will hole it in place. Use bottle valve to modulate flow. This was a 1970' carb set up procedure for some Chrysle vehicles that ran Qjets and for some Carter carbs too. (They had a fancy bonnet and valve to do the same thing as the clothes pins and cylinder valve). Don't smoke and do wear your safety glasses...... If car will idle on propane then the carb needs work, if not then the engine needs further diagnosis. 

Good luck. Flame on. Ladd


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