Quadrajet - Simple Power Adjustments - Pontiac GTO Forum
User Tag List

  • 7 Post By PontiacJim
  • 2 Post By bigD
LinkBack Thread Tools Display Modes
post #1 of 7 (permalink) Old 01-17-2018, 01:27 PM Thread Starter
PontiacJim's Avatar
Join Date: Dec 2012
Location: Gastonia, NC - Born & raised in Connecticut - 31 years
Posts: 3,454
Mentioned: 0 Post(s)
Tagged: 1 Thread(s)
Quoted: 347 Post(s)
Quadrajet - Simple Power Adjustments

Thought I would share some thoughts on Quadrajet adjustments as I remember doing in the late 1970's/early 1980's when I had my GTO's and used to tinker on other Pontiacs - and learned from reading magazine articles and hands-on work seeing we did not have the great resources today found on the internet. So some of the things I did may not have been the "correct" or "approved" way to do things, but some did work. Here is one of the things I learned to do in adjusting the Secondary Air Valve on a Q-jet. I knew nothing about accelerator pump rod placement or getting a better shot of gas to the primaries, but I included it as today I know it can cause a "stumble" or hesitation along with the Air Valve opening.

In my younger days, the Quadrajet was known as the "Quadrabog." Often it was a Secondary Air Valve problem. If the Secondary Air Valve is not opening, opens to early, or opens to late, it can be adjusted. If the Primaries are not getting a good solid shot of gas when you open them up, you could get more air than air/fuel and find a "bog/stumble."

I identified those parts I am referencing by marking them with "red." I also recall on most carbs I disabled the vacuum diaphragm linkage rod that connected to the Air Valve. I don't think I knew what it did or its purpose other than it held the Air Valve tightly closed when the engine was running and it didn't seem like that was a good thing for performance. LOL I added info on the diaphragm and its operation near the end. But either way, the Secondary Air Valve opening rate can be adjusted to perform at its peak and eliminate any bog/hesitation you might experience and wonder what it is or if you need a new/rebuilt carb.

First, to get the best shot of gas from your accelerator pump, make sure the accelerator pump rod is in the hole closest to the carb body as shown in Pic#1. To adjust it, you will have to drive the small pin through the pin boss that holds the pump lever in place - Pic#2. Do not break the pin boss or the carb will be junk, you should not have to use a lot of force. Drive it through with a small punch just far enough until the pump lever will slip out. Insert the pump rod into the inner hole and then set the pump lever back into place. I usually use a small screw driver behind the pin and pry/push it back into the pin boss. This setting will ensure a good fast pump shot of gas (providing your accelerator pump is in good working order & not your problem). Some early Q-jets had spring clips holding the pump rod in place, so you can pull these off and move the rod without knocking out the pin.

Another trick is to grind 1/8" off the top of the accelerator pump which will seat the accelerator pump higher in the gas well and provide a longer stroke/shot of gas. But, the carb top will need to be removed to do this.

Secondary Air Valve. The Secondary Air Valve has a lock. There are 2 different styles of locks depending on the year of the carb.

The earlier carbs have a lock at the top of the carb that works off the choke lever and will not allow the Air Valve to open until the choke is fully open - Pic#3. If the choke is not fully opening for whatever reason, it can keep the Secondary Air Valve shut and you won't have much power added by the opening of the Air Valve. It is very visible and easy to find. I like to remove these completely rather than try to adjust them. Like the pump lever, it is held in with a pin. Remove the pin and remove the lock and put it in a safe place if you want to keep the carb original.

The later carbs have a different design that locks the choke closed and still utilizes the choke operation and ties into the Fast Idle Cam function. It is found on the side of the carb - Pic#4. Again, you can go through the process to adjust the lock as shown in the diagram, but its operation can still be problematic and rather than experience any problems like sticking, I disable the lock. The pin that goes through the throttle shaft and is part of the lock operation can be bent outward with a heavy screw driver just far enough so it will not engage with the locking arm. I believe I have also disabled it by removing the lock arm altogether, bending the lock arm out of the way, and even cutting the pin, but these were in rebuilding the carb and the parts were disassembled - and keeping things original was not part of my thinking by any means.

The next thing, and what often give the Q-jet its "Quadrabog" name was adjusting the Secondary Air Valve rate of opening. If it opened too soon, the engine gulped in more air than air/fuel and the engine would bog down losing power momentary, and then pick up again and start pulling (sometimes called "nosing over" or "stumbling"). By then the guy you were racing pulled away as you tried to play "catch up." The other side of the coin was that the Secondary Air Valve did not open soon enough so you gave up some of your engine's power until it finally popped open and the guy you were racing pulled away as you tried to play "catch up."

Fig#5 shows the adjustment for the Secondary Air Valve opening rate. There is a spring that can be adjusted to increase or decrease tension on the Valve to either allow the Air Valve to open sooner or later. I used to get the spring adjusted so it would allow the Air Valve to pop open the soonest and yet not to the point where the engine would bog down because it opened to soon and sucked in a big gulp of air. The diagram calls it the "Wind Up Spring" and there is a table included for initial spring tension settings based on your carb number. Well this is fine for your initial settings, but the "seat of your pants" adjustments will put the spring tension where it best opens the Air Valve for your engine/car application. it is very easy to adjust and you can do it out on the road to dial in the Air Valve.

The adjustment is found on the passenger side of the carb. All you need is a small bladed flat screw driver that fits the adjusting screw and a small allen wrench that locks down the adjustment screw. You will be looking straight at the small screw which is found on the outside of the carb, while the locking allen screw is found up under the casting.

To adjust, first back off the allen screw that locks the adjusting screw into place. It should not take much effort to break it loose. You do not have to back it out, just loosen it enough so that you can turn the adjusting screw freely. So I usually have my screw driver blade in the screw slot and put a little pressure on it (as if to tighten or loosen the screw) while loosening the locking screw. I turn the allen wrench to back off the locking screw just enough until my screw driver can turn the adjusting screw fairly easily. You are now ready to adjust the Air Valve.

As I recall, if you turn the adjusting screw to the right, you will be adding more tension to the Air Valve spring - keeping the Air Valve closed longer. Turning it left will remove tension - allowing the valve to open sooner. I used to loosen/back the screw off until you see the Air Valve begin to open. Then turn the screw the other way until it fully seats and closes again. Once it seats, stop. This setting is the lightest you could set the spring, but it will most likely be too light for most applications and when you put the gas pedal to the floor, the Air Valve will open too soon and that's where you will get the "bog" from the carb. You can look up your carb number and find the spring tension setting. This will be how much more you will now add to the spring tension screw after you have gotten the Air Valve seated. If you have a 1968 Q-jet for example, you would add another 1/2 turn to the spring tension screw, hold it in position, then tighten down on the allen wrench to lock the spring screw. You don't have to tighten down real hard- its a small screw and you do not want to strip the threads out of the carb casting. If you cannot turn the spring tension screw, the set screw is tight enough.

Once adjusted, you can take your car for a road test and evaluate the Secondary Air Valve opening when you put the pedal to the floor. It take a few seconds for the Secondaries to open and you can usually hear the change in tone so you know they have opened up. If you feel the carb/engine "bog" down when they open and then feel the car pick up power again, you need to tighten the air valve spring a little more. I keep the allen wrench & screw driver with me when I do my road tests and adjust the Air Valve on the side of the road. Only tighten the adjusting spring screw 1/4 at a time until you eliminate the bogging and the engine picks up power smoothly. Then you are finished.

On the other hand, if you test the carb and you don't feel any bogging or engine hesitation, you can loosen the Air Valve spring 1/4 turn at a time until the engine bogs down on you. You now have adjusted the Air Valve spring too loose, so just tighten it back up 1/4 turn and you should have your Secondary Air Valve opening rate tuned in for your car.

Your settings will be different based on your engine build, manual transmission or automatic, rear gearing, and sometimes the weather. So you may find the carb works perfect today, and tomorrow you open it up and you get a slight bog/hesitation. You can adjust the spring tension and give it an additional 1/4 turn or see if the bog/hesitation goes away. It is one of those adjustments you can play with and better know your car.

There is also a vacuum diaphragm on the side of the carb called a "choke pull-off" which also affects the rate of the Secondary Air Valve opening. Cliff Ruggles Rochester Quadrajet book explains their operation and the adjustments that can be made. These diaphragms hold the Secondary Air Valve shut by using engine vacuum. When the engine loses vacuum as you put the gas pedal to the floor, the diaphragm looses vacuum and allows the Secondary Air Valve to open at a controlled rate rather than pop open wide open using just the Air Valve spring tension alone . These diaphragms have specifically sized inlet hole to control the rate of release and the rate of speed the Secondary Air Valve opens and is designed to work together with the spring tension adjustment of the Air Valve. These can sometimes be restrictive and not allow the Air Valve to open fast enough in some applications. Cliff's book says to test them, with engine off/no vacuum, compress the diaphragm fully and then release it. It should take about 2-seconds to fully release. Some will take longer and these should be reworked to get closer to the 2-second release rate.

There is a small measured diameter hole found in the tube going into the diaphragm and to which the rubber vacuum hose connects to. The size of the hole is a measured restriction (like a carb jet) that controls the rate of the release of the diaphragm arm - and the rate at which the Secondary Air Valve will open. If it is too small, as some late model carbs had, the rate will be slow and the Secondary Air Valve's full opening will be delayed longer. Cliff recommends drilling open the holes from .016" to .022" to get the diaphragm release speed to 2-seconds.

Personally, I used to make the diaphragm inoperable by simply removing the linkage rod that goes from the diaphragm to the Secondary Air Valve. - its fairly long and you can't miss it. Then I would adjust the Secondary Air Valve spring tension as above. Never had any issues doing it this way, but Cliff's book states that this can over stress the Air Valve adjustment spring - and it might in certain conditions when perhaps bigger HP engines are involved. I always did my tuning on factory engined cars, not anything highly modified. I never ran into any concerns or problems with Air Valve springs or their adjustments and doing it this way "popped" the secondaries open quickly as opposed to a controlled and smoother air valve opening regulated by the vacuum diaphragm or "Choke Pull-Off."

So the "Choke Pull-Off" diaphragm can also be used as a means to adjust the Secondary Air Valve rate of opening and you might want to check your diaphragm's rate of opening. Or, you may want to disable it as I have done in the past, and adjust your Air Valve spring tension without it by removing the linkage rod going from the diaphragm to the Secondary Air Valve and see what you think.
Attached Thumbnails
Click image for larger version

Name:	01  Accelerator Pump Lever.JPG
Views:	116
Size:	57.1 KB
ID:	102065   Click image for larger version

Name:	02  Pump Arm Pin.JPG
Views:	138
Size:	41.3 KB
ID:	102073   Click image for larger version

Name:	03  1st Design Air Valve Lock Out.JPG
Views:	140
Size:	121.4 KB
ID:	102081   Click image for larger version

Name:	04  2nd Design Air Valve Lock Out.JPG
Views:	131
Size:	221.0 KB
ID:	102089   Click image for larger version

Name:	05  Q-Jet Spring Windup.JPG
Views:	115
Size:	165.6 KB
ID:	102097  

PontiacJim is offline  
Sponsored Links
post #2 of 7 (permalink) Old 01-17-2018, 04:16 PM
Former Moderator
Join Date: Oct 2014
Location: Lake Arrowhead, Ca.
Posts: 1,499
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 106 Post(s)
Bookmarked, thanks for posting.
Goat Roper is offline  
post #3 of 7 (permalink) Old 01-18-2018, 10:21 AM
1968gto421's Avatar
Join Date: Mar 2014
Location: Travelers Rest, South Carolina
Posts: 752
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 70 Post(s)
X2, Thanks Jim.
1968gto421 is offline  
post #4 of 7 (permalink) Old 05-06-2019, 03:14 PM
Join Date: Aug 2018
Posts: 54
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 17 Post(s)
Quadrajet - Simple Power Adjustments

Hi Jim,

So if I understand you correctly, the choke pull off is kind of an unnecessary thing if the desired setting can be achieved by using the adjustment screw as you described?? The reason I ask is that I am buying a 68 GTO long distance and the guy who checked it out for me commented that it "needs a pull off choke". Can you explain for me please? Thanks in advance!!

jayvee53 is offline  
post #5 of 7 (permalink) Old 05-06-2019, 08:31 PM Thread Starter
PontiacJim's Avatar
Join Date: Dec 2012
Location: Gastonia, NC - Born & raised in Connecticut - 31 years
Posts: 3,454
Mentioned: 0 Post(s)
Tagged: 1 Thread(s)
Quoted: 347 Post(s)
Originally Posted by jayvee53 View Post
Hi Jim,

So if I understand you correctly, the choke pull off is kind of an unnecessary thing if the desired setting can be achieved by using the adjustment screw as you described?? The reason I ask is that I am buying a 68 GTO long distance and the guy who checked it out for me commented that it "needs a pull off choke". Can you explain for me please? Thanks in advance!!


OK, first you need to know what the seller is calling a "pull off choke." Is he referring to a linkage piece or the vacuum diaphragm (technically called the "vacuum break).

The secondary opening I have referenced is the removal of the linkage rod that goes from the vacuum break to the secondary flap lever.

The linkage that connects the vacuum break to the secondary air valve flap is bent in such a way that it is part of what is called the "choke unloader system." As the linkage rod is pulled forward once engine vacuum is developed when the car starts, the linkage rod contacts a tang connected to the choke. It pulls on the tang, which linked to the choke, pulls open the choke just slightly to let air in and prevent flooding.

In removing the linkage rod from the vacuum break & secondary air valve flap, this disables the use of the vacuum break and its function upon the secondary air flap which is to keep the air flap closed when engine vacuum is present or closes it shut after opening when you let off the throttle and engine vacuum returns. Under wide open throttle, there is no vacuum going to the vacuum break to keep it closed. The spring tension on the secondary flap along with the engine drawing in air to further aid in its opening, pulls on the vacuum break diaphragm arm to extend it out because there is no vacuum to pull the arm in and its linkage rod used to pull the secondary air flap closed. The vacuum break is essentially made inoperative without vacuum.

Remove the linkage rod altogether, and you have made the vacuum break useless, or inoperative. Without the vacuum break regulating the opening rate of the secondary air flap, or pulling the flap closed once you let off the gas, you now want to manually adjust the opening rate of the secondary air valve using spring tension only. You are not concerned about the closing rate because it will simply close shut with the spring. It is the opening rate that requires adjusting and this is where you fine tune the opening rate to your particular engine/car.

With the linkage rod removed, you have now disabled the function of the "choke unloader system" tang. So you would want to now manually adjust your choke so the flap is set slightly loose or even just a hair open to compensate for what the choke unloader does. Generally, when the engine is cold, the choke will close a bit on the tight side. The choke unloader system uses the linkage rod you just removed to put enough tension on the choke tang (as the vacuum break pulls closed) to crack the choke open just a little so air will enter the carb and the carb does not flood when first cranking the cold engine.

With all this said, using and keeping the function of the vacuum break, secondary air flap linkage rod, and choke unloader system won't hurt a thing and may be best for most people. The vacuum break can be made to respond faster by selecting a different canister or drilling out the inlet hole and additionally adjusting the tension spring on the secondary air flap - just may require a little trial and error to get it where you want it.

For myself, I have always removed the linkage rod and manually adjusted the secondary air valve spring tension to where I found it worked best - popped open with no bogging. It would take a few runs down the road and I would have my needed tools with me. Never had an issue with the choke as I would adjust it manually anyway with my concern being that it was fully open when the car was at full operating temperature.

Cliff Ruggles' book recommends using the vacuum break and try finding one that matches your engine/car with regards to opening rates. He also does not recommend using just the spring tension only to adjust the secondary air flap as it will/can weaken the spring. I like to adjust the carb my way as described which goes against Cliff's recommendations. I am no expert and respectfully feel Cliff is. I sometimes also run with scissors and Cliff probably would not.

The vacuum break or any missing linkages can be gotten, so not really a big deal in my book. Hope that answers your question and did not confuse you further.
PontiacJim is offline  
post #6 of 7 (permalink) Old 05-06-2019, 08:47 PM
Join Date: Aug 2018
Posts: 54
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 17 Post(s)
Gotta say this is going to take a little time to digest......Thanks for taking the time to respond and explain...I'll certainly study it further!
I just assumed he meant the linkage piece.
jayvee53 is offline  
post #7 of 7 (permalink) Old 05-07-2019, 06:28 AM
Join Date: Jul 2016
Location: North Louisiana
Posts: 524
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 54 Post(s)
I always used the vac break, pull-off, or whatever you wanna call it, long before I ever heard of Cliff Ruggles. It insures that you get a SMOOTH air flap opening.

This is especially important if you are running an auto trans, plus a stock converter. It's real easy to get a bog, if the flap opens too quick.

Now if you have a manual trans, or a high stall converter, a super quick opening rate may be just fine for you.

I ran a stock 13" converter, in all our 455 bracket cars. Launched from an 800 rpm idle. So a smooth, gradual air flap opening was mandatory. That, along with the full pump shot which PJ described, provided us with just what was needed for lots of bracket racing wins.
geeteeohguy and PontiacJim like this.
bigD is offline  
Sponsored Links

  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, LeMans & GTO Engine Tuning and High Performance

Quick Reply

Register Now

In order to be able to post messages on the Pontiac GTO Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode

Similar Threads
Thread Thread Starter Forum Replies Last Post
Power: It can't really be this simple? Bredfan 1964-1974 Tempest, Lemans & GTO General Discussion 8 11-01-2017 09:04 PM
carb adjustments simmons68 1964-1974 Tempest, LeMans & GTO Technical and Electrical Wiring 2 05-17-2010 08:12 AM
Power Windows, Power Locks, Seat Belts, Power Rear View Mirror, Back Speaker Location GQUINCOCES 1964-1974 Tempest, Lemans & GTO Convertible Top, Interior, Glass and Trunk 2 01-03-2009 07:18 PM

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

For the best viewing experience please update your browser to Google Chrome