Quadrajets – most stock 4-barrel Olds’s come equipped with one, yet most of us are reluctant to touch them, even when it comes to a simple rebuild. Why? Mostly because it is the least talked about, least understood carburetor mounted on cars. 4M, 4MC, 4MV, M4M; all these silly numbers, lots of levers, vacuum lines, and other gizmos all contribute to make that thing look overly complicated and impenetrable.
“Quadrabog”, “Quadraslob”; these and other names which I can’t mention, all are used to describe what is actually a good performance/mileage carb. Most of the Q-Jet’s bad reputation has resulted from the hot rodder’s “bolt on” approach and the availability of the popular Holley carb.
So if the Q-Jet can be made into a good performance carb, how do we start? Well, we first must understand a little about how they are different, and then we can use this knowledge to extract the excellent performance and driveability that the carb is capable of.
Quadrajets carb be either a 4MV or 4MC. The nomenclature only indicates the choke configuration. The 4M has a manual choke, the 4MC has a choke on the carb (’70 and later Olds) and the 4MV has the choke housing on the intake (’66-69). The M4M is the emissions Q-Jet introduced in 1975, which has had certain adjustability added in while some of the off-idle power transition is taken out. It can be made to deliver power, but the older models will do a better job.
Basically, the carb is almost totally vacuum controlled, proper setting of the signalling devices is ABSOLUTELY NECESSARY to ensure it will work correctly. This should be done BEFORE ANY CHANGES ARE MADE! If you are going to rebuild the carb, do this before you set it up.
For the rest of this article, we will assume that you are in the process of rebuilding the carb as much of what you can do is done with the carb apart. Buy a quality rebuilding kit and clean everything thoroughly after disassembling the entire carb. Lay all the pieces out and familarize yourself with all of them. I suggest that you have the Olds Chassis Service Manual for your year carb or an aftermarket book on Quadrajet rebuilding. [Rebuild kits usually have excellent diagrams included.]
We’ll go through the rebuilding steps while we explain how the Q-Jet works. This way, you’ll learn how to fix it as well as what affects what. OK, let’s get started.
With the carb disassembled, we have three major pieces: the base, the float bowl, and the air horn (top of carb). Put aside the base and the air horn for now and grab the float bowl. Most of what we are going to do is going to happen in this baby, so start by turning it upside-down and looking at the bottom.
FLOAT BOWL TURNED OVER: Arrows indicate main fuel casting wells. These MUST NOT LEAK [famous leaking wells] as they will draw raw fuel into the manifold and ruin performance and idle.
You’ll see four bumps sitcking up. These are the casting wells for the main fuel wells. These are capped when the carb is make, but after a while, they tend to crack and leak. Mix up some epoxy and coat the tops of these bumps, covering the pressed in plugs. Set aside the float bowl and let the epoxy dry.
Pick up the carb base and run a straight edge over it [both surfaces mating to manifold and float bowl]. If it is not straight, throw it away. Go junk yard jaunting and aquire a bunch of used Q-Jets (I should have told you this earlier) for similar years [good luck] (’66-67), (’68-69), (’70-72), (’73-74), (’75-80). Check for straightness and also check to see if the throttle butterflies open without sticking or close properly. The secondaries should open to about 5 degrees of vertical, no more, no less. Put the base aside and pick up the now dried float bowl.
Turn the bowl right-side-up and peer into hole “B” in the picture at the top to the page. This is the power piston bore. If you look real good you’ll see a little spring in there that you didn’t take out when you TOTALLY disassembled the carb. You’re forgiven, but use needle-nosed pliers and remove it. This baby controls the idle/off idle fuel mix, and it is not happy if you have changed the cam to a long duration type. This is especially true for 350/403s. Guys who have went the W-31 route, TAKE NOTICE!
There are many power piston springs to choose from, but some of the favorites I’ll suggest are:
7037851 - 72 W-30 very soft (lean) 7029922 - 69 W-31 soft/medium 7011967 - 69 W-30 medium 7036019 - 70 W-30 auto stiff (rich)
Install the leanest first, run the car, and then try the next, and so on. Stay as lean as possible here as these springs affect idle more than power.
Next, we’ll work with the primary metering rods. These are the skinny little guys which are stuck in the primary metering jets.
They’re attached to the power piston (which fits into the hole you dropped the spring in). No matter what set you have, go out and get the following:
7034840 - stamped 40B (richest) 7034842 - stamped 42B 7034844 - stamped 44B (most Oldsmobiles) 7034846 - stamped 46B (leanest)
Again, start with your present rod and move richer to see if performance improves. If the car is “flat” on acceleration try leaner, then richer.
The next step is the primary jet. These are the screw in babies in hole “D”. Here’s the ones to have in your tool crib:
7031969 - stamped 69 (leanest) mostly in 350s 7031970 - stamped 70 1970 350 & 455 7031971 - stamped 71 1966-68 all 7031972 - stamped 72 1968-69 400 SMT 7031973 - stamped 73 7031974 - stamped 74 1968-69 W-31 7031955 - stamped 75 1969-69 400 W-30/W-32
After getting the best results with the rods, start with one step leaner jet (unless you already have a 69) and work up.
Obiously, we’ve been combining rebuilding with tuning in this article. In reality you’d be rebuilding to stock specs first, trying the carb, then going back inside and making changes. This would hold true unless you had changed to a radical cam at the same time. Then you should have replaced the power piston spring with a leaner unit. This is because the power piston is controlled by vacuum and big cams make less vacuum, hence a softer spring. The steps from now on SHOULD BE MADE NO MATTER WHAT.
First, the fuel inlet seat (“A” on the top view of the carb) should be replaced with GM part 7035140, which should be used with all stock/modified cars using GM mechanical fuel pumps. With any fuel delivery system with over 5 psi, use GM assembly 7035130. Stay away from the seat supplied with most rebuilding kits unless rebuilding a totally stock carb as they are of questionable quality and do not deliver sufficient fuel.
Set the float to 9/32″ and ensure that the fuel level does not climb too high and slosh into the power valve hole and overly enrich your mixture.
FLOAT BOWL PIVOT PIN – Held in place by the air horn. Spread it slightly so that the floar/needle can’t climb out under high fuel pressure situations.
Adjust the float, set it and the needle in place, then insert the primary needles/power piston in place. BE CAREFUL TO INSERT THE NEEDLES IN THE JETS AS YOU INSERT THE PISTON OR YOU WILL BEND THEM! Drop in the float chamber baffle as it keeps the fuel in the float well and has no affect on the amount of fuel in the chamber!
Assemble the air horn and the fuel float bowl using the proper gasket. There are quite a few different ones so make sure the one you use has the same holes and the same configuration as your carburetor. This operation is usually a pain as the accelerator pump lever arm link requires the arm to be inserted into it as you are trying to tilt the air horn onto the float bowl in 1968 and later carbs. The ’67 and ’66 units use a link which is inserted afterwards and has a pin to hold it in place. Scrounge the junk yards for this piece and use it. CAUTION; some links are of different length, be sure to use the one with the same length.
For proper accelerator pump discharge (Olds engines) always use the inner lever arm position. screw down the carb air horn with the two center screws. STOP HERE and turn the carb over. Check the gasket used between the float bowl and the base as you did for the air horn/float bowl unit. Place it on the base and mate the base to the float bowl. You will use either two screws or three, depending on the year and type of carb you have. I suggest you utilize small lock washers on these screws. Use ones which will fit in the screw well. This is a better method than Loc-Tite. These screws have a nasty habit of loosening up and causing vacuum leaks.
Now turn the carb over, install the rest of the screws and turn them in according to the sequence in the Chassis Service Manual.
Now it’s time for the secondary metering rods. We are able to control fuel metering by both the rod size and the hanger. Let’s discuss rod size first:
7033830 - stamped AY (leanest) '73 and later 350 7033658 - stamped AT '70 442 SMT & 350 7033655 - stamped AU '66-69 big block, W-30 & W-31 7046004 - stamped DA 7033549 - stamped AX '71 Toro 17056618 - stamped DS (richest) Buick V6 Turbo
Most carbs can be enriched one rod with no problem. All W-Machines should go right to the “AX” rod. The “DS” rod should only be used for racing.
More adjustment of the secondary circuit can be made with the rod hanger. These hangers are coded A, B, C, etc., in 0.005″ changes in height, with “A” being the highest and “Z” being the lowest. Therefore, an “A” hanger will pull the rods out sooner and higher than a “Z”. These are no longer available except for the “V” hanger, so off you go to the junk yard.
OK, now that you’ve acquired all these pieces, rebuilt the carb, installed baseline rods, jets, and the like, you are ready for final testing. Adjust all the fast idle, dash pot, vacuum break, etc. as specified in the chassis service manual [or the detailed kit instructions]. Now you can bolt the carb to the manifold. I suggest that a relatively SOFT gasket be used. Fel-Pro and Mr. Gasket offer a thicker unit with a composition which seems to hold up well. If this gasket gets too hard from heat (a common problem with aluminum manifolds), it will leak and ruin your idle/tuning. Remember, a Q-Jet is vacuum controlled, so……
I suggest that you run an in-line fuel filter of very high fuel flow capacity. A Fram GF-15 is what I use. If you are a stickler for stock fuel line appearance, put it in-line where the line comes out of the front of the K-Frame and attaches to the fuel pump. The reason for the in-line filter is that we want to remove the filter in the nose of the carb. This allows us to almost double the fuel available to the carb during wide-open throttle situations.
Our two final adjustments are critical. The first is the secondary air valve spring “wrap”. This little guy determines when the secondaries open. While I will give you the base-line, you’ll have to find the optimum by testing (PEDAL TO THE METAL!).
The allen head set screw (A) must be loosened first. When there is no tension on the air valve butterflies, the spring tension is set with the screw (B).
Loosen the allen head screw until you see the tension go completely out of the butterfly. Then insert a small screw driver into the screw (B). Slowly turn the screw clock-wise, while tapping on the carb lightly until the butterfly rotates up and contacts the air horn. Turn the screw another 1/4 turn. Hold the screw in this position while tightening the allen head screw. (Wouldn’t it be nice to have a third hand now?)
This is your base-line setting. Less tension will opent he valve sooner; more will open it later. The test should go like this:
1. Disconnect the vacuum hose from the choke diaphram and plug the hose ONLY.
2. Go out and try a Wide Open Throttle acceleration. (The fun part!) Decrease the wrap (loosen) until a loss of performance is noted by bog.
3. Connect the diaphram and see if the bog goes away. If not, tighten in 1/8 turns until it does.
4. Once that is as good as possible, go one step richer with yor secondary rods and see what happens. If you run betteer, go a step richer – if not, check the wrap again. If there is no improvement, go back to the old rods.
5. When you have the old rods back in, go two steps towards “A” (i.e. from a J to H) with the rod hanger. Repeat the procedure.
6. After you get the best results, check your plugs to see if you are too lean. If so, go up two numbers in your primary jets and one step in your primary rods. Here’s more information on the jets, metering rods and hangers. Here’s page two on secondary rods.
7. If you are still too lean, consult below.
The following racing mods should only be done for a car which exhibits overly lean conditions @ WOT or is highly modified and is a RACE ONLY CAR.
Mike Jones Carburetion, 7602 Talbert, Hunnington Beache, CA 92647 sells accelerator pump pistons with differing length shafts. These allow more or less of a shot of gas. He also offers a ligher return spring and a stiffer shaft spring. Mixing/matching can allow more precise metering. The shaft length also allows a different point at which fuel delivery starts.
The air valve dashpot can be modified by drilling out the orfice from 0.025″ to 0.035″. The rear fuel bowl vent can be plugged to prevent fuel sloshing. ’75 and later Q-Jets ought to be enlarged to 0.010″.
The passage between the main fuel well and the two secondary wells can be enlarged to 0.050″, which will allow them to fill quicker.
Here are some DON’TS!
Don’t remove the baffles from the secondary bores. They ensure proper fuel mixture/atomization.
Don’t remove the plastic float chamber baffle as it helps to direct the fuel into the chamber as it sits above the fuel level.
Don’t try to set the idle without holding the hot idle compensator tang back on the back of the carb.
Follow these tips and your Quadra-Jet will perform with the best of them. Because it is mostly vacuum controlled, it does require a bit more setting up than a Holley, but when properly dialed-in, it can deliver btter around town driveability and real Go Power when the hammer is dropped.
Try these tuning/adjustment tips. They have worked very well for me. I only have stock cars and so haven’t tried the highly modified stuff.
I get as many old Quadra-Jets as I can from the junque yarde at $3-$5 each. This is a source of parts. Most all of the carbs are interchangeable as well as their 3 main parts.