The Rochester Quadrajet is one of the most common four barrel carbs out there; it was used in millions of GM and other vehicles, from the late 1960s through the late 1980s.
It is a great carburetor, but it has a reputation for problems - including a stumble or bog under hard acceleration. This is usually caused by an out of adjustment secondary air valve - and is easy to fix.
The Q-Jet's signature feature is its huge secondaries, which are controlled by engine vacuum - and spring tension. If the tension on the spring that controls the rate at which the secondary flaps open is not right, the result will be premature opening of the secondary air flaps, which causes the full-throttle stumble that plagues many out-of-adjustment Q-Jets.
To make the adjustment, you'll need to remove and reinstall the air horn (the top of the carb casting). This looks a little intimidating but it's not difficult, if you are patient and careful.
First, locate the accelerator pump rod & arm on the driver's side of the carb. The arm (usually, it's a greenish piece of metal is held in place by a press-in fitting that must be eased out partially (not all the way!) in order to let the arm pop loose and hang free. Use a small punch or equivalent to slowly/gently drive the press-in fitting out just enough to let the arm pop out/swing free. Do not push it out all the way - or remove it - or you will have a chore on your hands getting it (and the arm) reinstalled.
Next, (gently) remove the little screw that holds the secondary metering rods in place (the hangar that holds the rods is that thingy between the air valve secondary flaps). Put them aside, being careful not to drop them (they are fragile).
Now remove the screws/bolts that hold the air horn to the carb. There are typically six screws (plus the two large front bolts that secure the carb itself to the intake) that need to be removed.
You will also need to remove the small set screw on the passenger side of the carb that holds the choke rod in place on the primary side of the carb. Put the little metal tab that connects the choke rod to the carb someplace safe where you won't lose it! Let the rod hang free.
Ok, now you are ready to - gently! - lift the air horn off the carb body. Try not to damage the gasket underneath as you do this. (If you do, replacement gaskets are easy to find - just be sure you get the exactly right one for your specific Q-jet).
Now you can turn the air horn over and look at the underside. On the passenger side, to the left of the big air valve doors, you will see a small rod and a spring with a hook that "catches" the rod. This is the set-up that controls the tension on the air valve doors. The adjustment screw is on the side of the casing; there's a small allen bolt underneath that locks the adjustment screw/spring tension in place.
To make the actual adjustment:
* Gently loosen (but do not remove!) allen bolt to allow the adjuster screw to be turned out/adjusted.
* Gently turn out adjustment screw to relieve all tension on the air valves.
* Before adjustment, make absolutely sure (visually verify) the little spring/hook is just contacting the rod, with no preload or tension whatsoever. Be sure the air valves are closed/seated fully as you do this.
* Now, carefully turn the adjustment screw in one full turn, then lock in place by tightening the allen bolt. (It may be helpful to carefully scribe or otherwise mark the adjustment screw in relation to the air horn body to absolutely be sure of "one full turn.").
This will get you close to where you need to be. Button up the carb and then road test the car and see whether the bog goes away. Each application is different, so it may be necessary to repeat the process and make an additional adjustments; do these in quarter-turn increments and note the results until you get it dialed in.
PS: The hardest part about reinstalling the air horn is getting it in place while keeping the primary side metering rods seated in their bores. They are spring-loaded and will "pop up" when you remove the air horn. Before you can reinstall the air horn, you'll need to - very carefully - re-seat the primary metering rods in their bores and then, using a small flatblade screwdriver (or thin knife) hold them in place as you lower the air horn onto the carb body. Do this with great care, being absolutely sure the primary rods are seated. They are very fragile and once bent, they're ruined.
Also: Don't freak about the choke rod on the passenger side of the carb. It is a little awkward to get back on, but it's not that hard provided you are patient.
Thanks for the air valve adjustment diatribe. Easier to go someplace when you have good directions. Our 73 Grandville bogs badly, and I was hesitant to tear into it- until this. The job went well, and when I got to the valve its adjustment wasn't even 'in the room'.
It's set, back together, and ready for a road test; but first I needed to credit you with a good job.
Thanks again.
Hi EZ,
You bet - and, keep us posted!
Originally Posted by EZPickins
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