Technical Information Bulletin Rev. AB 9-26-07
How to Tune a Q-Jet
(basic)
by Lars Grimsrud
Lafayette, CO
This tech paper will discuss basic set-up and tuning of QuadraJet carbs for optimum street performance and drivability.
The procedure outlined here differs from other I have seen, and is based on my years of experience doing this work in the quickest, least painful, most economical way. It is recognized that other people will have different methods of doing things, and may disagree with specific methods and procedures that I use.
Overview
The Rochester QuadraJet, in its various forms and configurations, has been used by various GM Divisions for various applications since the mid sixties. The last passenger car version of the carb appeared as an ECM-controlled carb in 1981 (1980 California Q-Jets were also ECM-controlled).
The Q-Jet is a highly versatile, tunable carb that will provide outstanding performance and reliability once set up correctly. This paper will discuss the tuning and setup, and will provide you with adequate data to make good decisions when jetting and adjusting the carb. This paper will not discuss basic rebuilding sequences, nor will I discuss operations involving machining operations and other severe alterations to the carb. There are many books on the market that deal with these subjects in depth. Rather, I will describe the various systems, their purpose, and a good tuning sequence to help you get each system and parameter set up correctly in the easiest way possible.
QuadraJet carbs have three basic tuning variables, and these get people all confused: Primary Metering Jet, Primary Metering Rod, and Secondary Metering Rod. Attempting to cure problems by tuning the wrong variable results in lots of frustrations for tuners and car owners.
These systems at times overlap in their operation. Not only does each system need to be properly tuned, but its timing and “overlap” with other systems is critical to proper performance.
When tuning, we think of each of these variables as controlling a different operating range: The primary jet size determines the fuel mixture at Wide Open Throttle (WOT). The primary metering rod determines fuel mixture at cruise speed and determines responsiveness of the idle mixture screws and off-idle performance. The secondary rods are a high-rpm compliment to the primary side, and are used for final “tweaking.”
On a Q-Jet, we see that we can control the fuel mixture throughout the operating range. This is different from a Holley: A Holley has a given main jet size which meters fuel throughout the rpm range, including cruise. At WOT, the power valve unseats, and opens a fixed orifice, dumping a fixed amount of fuel in addition to the main jet. Crude, but simple and effective.
The Q-Jet meters fuel through the main jets. Metering rods, suspended from a power piston, “plug off” part of the area of the main jets by being inserted into the jets. These rods have a “fat” diameter and a “skinny” diameter: The number stamped into the side of every metering rod is the “fat” diameter indicated in thousands of an inch. This part of the rod is pulled into the main jet at cruise, at idle and at other high-vacuum operating conditions (light throttle). It produces a lean operating condition for good fuel economy and good throttle response. When engine vacuum is lost, indicating a high-power condition, the rods are pushed out of the jets by spring pressure, and only their “skinny” tips, or power tips, remain in the jets. This richens the fuel mixture for peak power. All primary metering rods have the same power tip diameter (.026”). This fact is crucial to remember when tuning: Primary metering rod sizes have no effect on WOT performance. (NOTE: Some post-1975 Q-Jets for truck applications have metering rods stamped with an “M” designation following the number size. The “M” rods have .036” diameter power tips, and are not suitable for performance tuning unless the tuner takes this larger power tip diameter into consideration when calculating resultant metering area at WOT.)
Identification
Q-Jets are identified by a number stamped into the Float Bowl casting on the driver’s side of the car just above the secondary throttle linkage. If the carb is a Carter manufactured under license from Rochester, the number will be inside a round metal foil tag on the driver’s side of the carb just above the primary throttle linkage.
Metering Area
WOT fuel mixture is controlled only by the main jet size. Performance at cruise and at idle is then controlled by the rods. We can establish each of these mixtures independently of the other by knowing and understanding the concept of Metering Area.
Jet and rod sizes are always referred to by their diameter in thousands of an inch. But fuel flow doesn’t “see” diameters: The fuel “sees” the total metering area. So we must convert the diameter into a resulting area. We remember that the formula for area is r2. Thus a jet with a diameter of .070” has a metering area of:
Radius = ½ diameter
Radius = .035”
.0352 = .00384”
Thus, the metering area of a #70 jet is 3.84 thousands of a square inch.
But wait! There is a rod inserted into the jet, so we must subtract the area of the rod. Let’s say we have a #40 rod in that #70 jet. The area of the rod is:
Radius = ½ diameter
Radius = .020”
.0202 = .00125”
Thus, the area that a #40 rod “plugs off” is 1.25 thousands of a square inch.
The resulting metering area of the #70/#40 combination is thus 3.84 minus 1.25. The total metering area is 2.59 thousands of a square inch. This is the metering area of this rod/jet combination with the rod fully inserted in the jet. In other words, this is the metering area at cruise speed and at idle.
To see the metering area at WOT, we know that all rods have a .026” diameter power tip (except as noted with the “M” series rods). So we run the same calculation for a .026” diameter rod inserted in the jet.
It is these numbers that we will use in all comparisons when making jet changes. We will use these numbers also to look at the percent differences in jet changes.
So that you won’t need to run around with a calculator, my Carb Listing in Table 1 shows the metering areas for every carb listing at both cruise and at WOT (assuming rods with .026” diameter power tips). The number is the metering area in thousands of an inch for a single jet/rod in the carb. This number is effectively how rich/lean the carb is really jetted, and you can directly compare these numbers to see how the various carbs were set up by the factory. By dividing one area into another area, you can see the percentage difference in the jetting.
Figure 2 is a table showing you what the metering area is for every possible jet/rod combination. Each grouping of jets starts off with the rod power tip diameter of .026” so you can see the WOT metering area of that jet size. It then jumps to the first usable rod size.
Tech Tip #1
Before you go trying to fix all the errors of the previous carb tuner, set your carb up to the stock spec for your carb part number. A carb jetted and set up to its stock specs will usually run pretty good on just about any application, and this gives you a good starting point. From there, you can start doing refinements as outlined in this paper.
The carb number on a Q-Jet is usually stamped into the bowl casting on the driver’s side of the carb in the area above the secondary throttle shaft. The number starts with either “70…” or “170…”. If the carb is a Q-Jet manufactured under license by Carter, it will sometimes have the carb number stamped into a foil circle on the driver’s side of the bowl just above the primary throttle shaft.
The carb listing (Figure 1) is a partial listing of popular Q-Jets that I have compiled over the years. It is not a complete listing of every Q-Jet carb. Most notably, I have very few of the truck carbs listed, yet there are many truck carbs running around on passenger cars. I also have not started compiling all the Cadillac, Olds and Buick applications in this chart, but I do update it at irregular intervals. For the latest version with latest updates, drop me an e-mail on occasion to make sure you have the latest chart.
Tech Tip #2
What has a greater effect on performance: primary or secondary jetting? I constantly see people swapping around secondary rods, trying to get the best performance out of their cars. The secondary rods are very easy to change, and since the secondaries are so BIG, the secondary metering has to be the most important, right?
Wrong.
Most Q-Jets are 750 cfm carbs. This is more airflow than most small block engines can ever handle. Yet, GM used Q-Jets on everything from Overhead Cam 6-cylinder Pontiacs and Buick V-6’s, to 500 cube Caddys. How?
The secondary airvalve on the Q-Jet effectively makes the Q-Jet a variable-cfm carb. The spring windup of the airvalve combined with the bleed-off of the choke pulloff diaphragm allow the secondaries to open only as much as the engine can handle. Thus, if the engine can’t handle all of the cfm, the secondaries simply don’t open all the way.
The primary side, however, is used throughout the rpm range. It is always in use, and provides the metering for the majority of the power produced by the engine. Let’s look at the scenario:
You’re at the stoplight. You bring the rpm up slightly against the torque converter – 1500 rpm. You’re on the primary side of the carb only, and this is what is producing all of your torque right now. The light changes, and you put the pedal to the metal. All of your torque at launch is being produced by the primaries only, as the secondaries don’t see enough airflow to open. The rpm comes up quickly: 2000, 2500, and now the secondaries might be starting to crack. Almost all of the air is still passing through the primaries, and the secondaries are now starting to compliment it just a tad. 3000, 4000 rpm, and the secondaries might be half-way open. The primaries are still providing most of the airflow and metering. 5000, 5500 and you hit redline just as the secondaries hit about ¾ open. Second gear, your rpm drops, partially closing the secondaries back up, and you’re back to sucking the majority of the air through the primaries once again.
So we see, the secondaries provide only a compliment to the primaries. The primaries provide the vast majority of the fuel metering, and primary jetting is absolutely the most critical to proper performance. You cannot compensate for poor primary jetting by re-jetting the secondaries. So we are going to concentrate on jetting the primary side for peak performance, and then we will set up the secondary side to provide a proper compliment to the correct primary jetting.
Tech Tip #3
How can you tell if an off-idle stumble is caused by a lean or a rich condition?
A carb running rich, as well as a carb running lean, can cause an off-idle stumble or hesitation upon acceleration. To narrow it down, tap the roll pin out of the accelerator pump lever by using a small pin punch or a small finish nail. I actually use a small, broken drill bit that’s just the right size. Using a hammer, gently tap the roll pin in towards the choke air horn wall. Don’t jam the pin right up against the wall: Leave just a little bit of a gap so you can get a screwdriver blade in between the wall and the pin to pry it back again. With the pin tapped out, remove the accelerator pump lever. I like to do this with the engine running so I won’t have any trouble starting the engine without the accelerator pump. Now, rev the engine a little with the throttle. Notice if the engine seems quicker and more responsive, or if the hesitation & stumble is worse. If the engine actually feels more responsive with the accelerator pump disconnected, you have a rich condition. If the hesitation is worse than before, you have a lean condition. If there is no change whatsoever, you have a non-functional accelerator pump.
To verify a suspected lean condition after this test, simply hold your cupped hand lightly over the choke air horn area with the engine running at idle, restricting the air flow. If the idle speed and idle quality momentarily increases, you have a verified lean condition. You need to select a jet/rod combination that will give you a little more Cruise Metering Area. Make these changes in less than 10% increments using the Figures provided in this paper.
Tech Tip #4
How can you tell if your power piston spring is too stiff and not allowing the power piston to “seat” at idle?
If your engine does not produce enough manifold vacuum at idle and/or cruise (due to a lumpy cam or other engine parameters), it is possible that the power piston is not being pulled all the way down to its seated position due to the power piston spring being too stiff. The result is that the car will run very rich at idle, and the idle mixture screws will have little effect or response. Idle speed may also “float,” with idle speed starting high and gradually decreasing until the engine stalls due to the engine getting “loaded up.” There will typically be a puff of black smoke out the tailpipes when you “flick” the throttle.
To test for this, pop the top off the carb, remove the power piston/rod assembly, and remove the power piston spring from its bore. Re-install the rod/piston assembly without the spring and put the carb back together. The carb will now run in the full-lean condition all the time, and you can actually test drive it in this condition. You can also test for this condition very quickly (although you cannot actually drive the car) by inserting a long pin punch or a small long screwdriver down through the vent tube: Angle the pin punch slight forward, and you will hit the top of the power piston. You can now depress the power piston and/or verify if it is pulled down into its fully seated position. Be very careful when doing this so you do not jam the punch or screwdriver through your float. If this clears up the idle, improves idle mixture screw response, and eliminates the black smoke when you flick the throttle, you need to install a softer spring. Edelbrock has a complete power piston spring assortment available. You can also get many of the springs from GMPartsDirect using the GM part number shown in the carb listing chart.
Tech Tip #5
How can you tell how stiff the power piston spring needs to be, and how can you tell one spring from another?
If you have a few springs of various kinds laying around, it is not readily apparent which spring is stiffer than another. You can arrange them and order them from softest to stiffest as follows:
Using your carb, or a junk float bowl from another carb, as a testbed, remove the carb air horn (the “top” of the carb) and remove the power piston and its spring. Remove the primary metering rods from the piston. Now, drop a spring into the power piston bore and install the piston. Find a Phillips screwdriver, and place the handle of the screwdriver on top of the power piston with the shank of the screwdriver pointing straight up. Use a screwdriver that is light enough to NOT compress the power piston and its spring, but close. Now, drop flat washers onto the shank of the screwdriver and keep stacking them up until the piston compresses the spring and seats in the bore. Count the number of washers it took to compress the spring and label the spring as a “6-washer spring,” for instance. Do the same with the other springs you want to test. You’ll end up with a comparative rating of springs, like “4-washer,” “6-washer,” or “10-washer” springs. You now know exactly how to arrange them from softest to stiffest.
But which one should you use? You’ll need a junk Q-Jet float bowl for this test, and you’ll need to have your engine in running condition.
Using a stripped down, bare Q-Jet float bowl, you’ll notice that there is a hole in the bottom of the bowl right underneath the power piston bore. This is the vacuum hole that applies manifold vacuum to the power piston. Hook up a long vacuum hose to a manifold vacuum source on your engine. Now, install a power piston spring from your arranged spring selection into the piston bore and install a power piston on top of the spring. Start your engine, and stick the end of the vacuum hose onto the hole in the bottom of the stripped down float bowl. With the engine at idle, the vacuum applied to the bowl should immediately pull the power piston down against the spring pressure and seat the power piston firmly in its bore. If the piston does not fully seat, you need a softer spring from your arranged spring selection. If you have an automatic, put the transmission in “drive.” Make sure the power piston stays seated.
If you really want to do some testing, you can string the vacuum hose into the car, and with an assistant, drive the car around and observe under what conditions the power piston starts to unseat: While you drive, have the assistant stick the vacuum hose onto the bottom of the bowl, and observe what the piston does under various engine loads. Make sure you have a spring that’s stiff enough to make the piston pop up when your engine is under load, yet soft enough to keep the piston fully seated at idle, at cruise and under light acceleration. This makes for some really fun testing, and the results will pay off in a precisely matched power valve spring for some outstanding throttle response.
Of course, if you buy the power piston spring assortment kit from Edelbrock, the springs will be identified and labeled as to their vacuum rating. Select and use a spring with a rating about 1.5” to 2” lower than the idle vacuum of the engine (in drive).
Tech Tip #6
The idle metering circuit on a Q-Jet is not an independent, stand-alone circuit. The idle mixture screws in the throttle plate receive their fuel through the main metering jets. Thus, a change in the main metering circuit (jets and/or rods) will affect the idle circuit. The idle mixture screws cannot meter more fuel than the main jets/rods will allow. Thus, if your Cruise Metering Area jet/rod combination is too lean, you may find that your idle mixture screws are ineffective. If your idle surges, is rough & unstable, and adjusting the screws seems to make no difference (but you can kill the engine by turning them all the way in), chances are good that your cruise metering area is too lean. You can verify this by running your mixture screws out to the point where additional turns have no effect on idle. Then cover the choke area of the carb with your hand. If idle speed & quality increases as you restrict the air flow, your jet/rod combination is too lean.
Procedure
Here is my recommended sequence and procedure for doing a basic Q-Jet set-up:
1. Set the float level.
You’ll be amazed how many people try tuning a Q-Jet without ever checking the float level. An incorrect float level can give you all kinds of symptoms and problems, so get this one set right off the bat. Also, many commercially rebuilt Q-Jets have brass floats. I do not recommend use of a brass float in a Q-Jet. Use the correct “NitroFill” float available from NAPA/Echlin. Part number for most pre-75 Q-Jets is 2-440. Part number for most 75-80 Q-Jets is 2-442.
You have to pull the top of the carb off to set the float level. With the top removed, remove the big phenolic spacer that covers the area around the needle/seat. Hold the float hinge clip firmly seated and push down lightly on the float where it contacts the needle. Measure from the top of the float bowl to the top of the float at the rear edge of the float. Float level should be .375” for a street-driven car using a 1968 – 1974 carb; you can run it at .250” for racing. Early Q-Jets (1968-1972) can be successfully run on the street with the high float level, but you may see some fuel saturation of the air horn gasket with associated gas fumes. Later carbs (1975 and newer) do not run well in street applications with the high float level – run the 1975 + carbs at .420” on the float level. Adjust the float level by removing the float and bending its lever arm. Never raise the float level by forcing the float against the needle/seat to bend it – this will damage the needle.
2. Determine main jet size.
If you have a stock engine, always start with the stock jet size for the carb number you are using and work from there. If you have the typical street modifications like headers, good exhaust system and a free-flowing intake, you can start with a main jet size 2 sizes larger than stock.
Since we want to work on the primary side only, we don’t want the secondaries interfering with the jetting process. Chevy Q-Jets have a secondary lockout lever on the passenger side of the carb right at the secondary throttle shaft. This lever is actuated by the choke linkage, and prevents the secondaries from opening when the engine is cold. I call this the “primary jet tuning lever.” Use a piece of wire or string to engage the lever with the secondaries so that the secondaries cannot be opened.
You now need to find a short flat stretch of road to test drive the car. You need to be able to measure time-to-distance and/or speed-at-distance. I usually find a repeatable stretch of road about 300 feet long. This gets me through 1st gear and into 2nd. Make two or three runs on the car through this stretch and make note of time and speed to distance. Also note the seat-of-your-pants feel of the car (it’s going to feel pretty slow with the secondaries locked out…).
I recommend making jet changes in less than 10% increments. Go to Figure 2 and determine your WOT metering area for your current jet size. This will be the metering area of the jet with the .026” rod. With this number, go to the Jet % Change Chart and find the closest metering area match in the left vertical Metering Area column (Use the “Area” column and not the Jet Size column. The Jet Size column can only be used on carbs that do not employ a metering rod, such as Holley and Weber.). Follow the row across until you get into the “green” zone and find the closest number to 10%, but not greater than 10%. Now go straight up until you get to the new metering area number. This is your target. Take this number and go back to Figure 2 and find the closest jet size that will produce this metering area with a .026” rod. This is the first jet size you want to try, and this will increase your fuel mixture by the percentage indicated in the chart.
Now, to keep your off-idle mixture unaltered, you also need to check your cruise metering area. Go to Figure 2 and find your old main jet & rod combination. Note the resulting metering area for this combination. Now, go to your new main jet size that you’re going to be using and find the rod needed to produce the same cruise metering area you had before. Use this rod with the new jet.
By doing this, you are now changing only 1 parameter at a time: WOT mixture only. Idle, off-idle, and everything else is now unchanged, and you will be able to see the results from the mixture change at WOT only. With the secondaries still locked out, run the car 2 – 3 times down the same stretch and record results. If the numbers get better, you’re going the right way with the main jet size. If the numbers are worse, you need to make changes to the lean side instead of rich. Repeat this operation until you determine the main jet size that produces the best numbers. On many stock cars, you may be surprised to learn that you end up with the stock jet size. You have now optimized main jets.
3. Determine main metering rod size.
NOTE: There are two different “series” of primary metering rods. Q-Jets up through 1974 (the “4MV” series carbs) use the early series rods. 1975 and later Q-Jets (the “M4M” series carbs) use the second series rods. Pre-’75 (up through ’74) Q-Jets use metering rods that are approximately 2.47” long overall (total length from the metering tip to the extreme top of the rod). 1975 and newer Q-Jets use rods that are about 2.40” long. You cannot interchange the two different rod series. The late style rods are also available in the “M” series rods, designed for truck applications. These have fat, .036” diameter power tips on them, and should not be used unless you re-calculate the resultant WOT metering areas and account for this in your tuning. For example: A regular ’76 Vette carb might have a 77/48 jet/rod combination with the correct .026” diameter power tip rods. This gives you a WOT metering area of 4.12 thousands of an inch. If you use a 48M rod in the same carb, you end up with a WOT metering area of only 3.63 thousands. This is the same as if you dropped the main jet size down to a size 73 with the standard-tipped rods. Keep these relationships in mind when playing with rods. Currently, there are no second series .026” power tip primary rods available from any source – all second series rods are “M” series truck rods, including those sold by Edelbrock.
When switching main jets around in Section 2 above, you were also swapping out metering rods to keep the cruise metering area unchanged. You did this to make sure that your off-idle throttle response remained unchanged so that the throttle response off idle did not affect the tuning results from the main jet re-sizing. Now, with your new main jets, your cruise metering area is exactly the same as it was before, but that’s not to say it’s right.
There are several indicators of correct cruise metering area. First, check out Tech Tip #5 regarding the idle circuit. This is a good indication of a lean condition. But here’s another good indicator of correct cruise metering area:
A Q-Jet, when set up with the correct metering rod for cruise & idle, will produce a slight hesitation upon acceleration if the accelerator pump is disconnected. Using a small pin punch or a finish nail, carefully knock out the roll pin securing the accelerator pump arm to the top of the carb. I do this with the engine running so I don’t have any trouble starting the engine without the accelerator pump. With the pump disconnected and with the engine running in neutral, “flick” the throttle just a little. If the engine actually feels more responsive with the pump disconnected, your cruise metering area is too rich, and you need to install a fatter set of rods. If you get a severe stumble, or if the engine dies, you’re on the lean side and need smaller rods. When the rods are correct for the jets in use, you will get a slight hesitation when the pump is disconnected.
Once you have set the rod size up like this, verifying both the idle as shown in Tech Tip #5 and using the disconnected accelerator pump, a road test is in order. If the car is a little “flat” on light acceleration, or if it has a slight “surge” at steady cruise, you need to richen up the metering area slightly. If it is smooth and responsive on light acceleration, and feels smooth at cruise, you have the rod size nailed down.
Again, use the charts to keep all changes limited to 10% at a time. This will prevent you from “over-shooting.” Remember, with the main jet size determined, your rod sizing is affecting idle, off-idle, light acceleration, and cruise. In most cases, when there are problems with stumbles, poor idle, idle speed that starts out high and then degrades, and surging at cruise, the rods are too big and are causing a lean condition. On the other hand, if the rods are too small, causing a rich condition, the throttle will feel “lazy” or “slow” when you rev the engine, and you may get a puff of black smoke with a hesitation when you “flick” the throttle. Correct rods will produce crisp, clean and instant throttle response.
4. Determine secondary rod size.
You are now finally ready to unlock the secondaries. But before you start changing the rods, you want to get the secondary opening rate set up. This is determined by the spring windup.
It is a very common “speed trick” to loosen the secondary windup spring so that the secondaries will open very quickly. This is the single most common cause of a severe stumble or hesitation upon acceleration or transition into the secondaries.
The secondary spring windup is adjusted with a small, slotted-head screw on the passenger side of the carb, right at the top of the carb on the secondary side. The screw head points right out to the side. 90 degrees from this, on the bottom, there is an allen-head lock screw that keeps the slotted screw from turning. If you have trouble seeing it, place a mirror under the area until you spot it. With a small slotted screwdriver holding the adjustment screw, loosen the allen screw about ¼ turn. This will allow you to turn the slotted adjustment screw. Counting the turns, allow the slotted screw to slowly unwind until all spring tension is gone. You can use your mirror to see the spring disengage contact from the pin lever underneath the air horn. If the spring tension was lost after only ½ turn, the windup was too loose. Bring the spring into contact with the lever. Note when it just barely touches. From this point, wind the spring up between ¾ turn and 7/8 turn. This is a good starting point, and will prevent any bogs or hesitations due to premature secondary opening.
Now, you need to adjust the secondary rod hanger height. You’ve read all about the different letter numbers for the secondary hangers, and how a “Y” hanger will make your car faster than an “M” hanger or whatever. Fact is, you can bend and adjust any hanger to any hanger height you want, so it doesn’t make a heck of a lot of difference what hanger you choose to use. Just get it set up right:
With the secondary airvalve held wide open and the secondary rods pulled all the way up, measure the distance from the top of the rear wall of the choke horn to the secondary rod hanger hole in the hanger. This distance should be 41/64”. Bend the hanger to adjust – you have to adjust each of the two sides independently. You now have a “performance” rod hanger.
With this set, you can now play with secondary metering rods. A common speed trick mistake is to always install thinner (richer) secondary rods. Some engines and carbs will produce a secondary “lag” if the rods are too thin. On about half of the engines I work on, I obtain better performance by installing fatter “non-performance” rods. Again, a quick road test is the only way to set this up, so go back to your 300-foot stretch and make a few runs with rods both richer and leaner. Once you have found the rods producing the smoothest secondary transition and the best numbers, you can start unwinding the secondary airvalve spring. Relax the spring tension in 1/8 turn increments until the car stumbles on acceleration, then tighten up 1/8 turn again. You have now determined the quickest secondary opening rate that your engine can handle, and your secondary mixture is set.
Note that secondary metering rods come in three different tapers: long tip, short tip, and medium tip (see Figure 3). Most of the available after-market metering rods have the long tips, and these will produce a full-rich mixture upon the slightest opening of the secondaries. Many street engines will produce better performance by using the short tipped rods. A short tipped rod does not allow a full-rich mixture until the secondaries are opened quite a ways, keeping the mixture a little lean initially. This can produce smoother and crisper performance in many applications. Next time you see a junk Q-Jet laying around, make sure you yank the rods and jets out of it: many old truck carbs have some really good short-tipped secondary rods in them. Figure 3 lists all the secondary rod letter codes, part numbers, and measurements.
Parts
If you don’t have a stash of used Q-Jets in your basement to rob jets and rods out of, you can get parts from Edelbrock. Your local parts store should be able to order them for you. Following is a partial listing of Edelbrock Q-Jet parts and part numbers:
Primary Metering Rods (pairs) for 1974 & earlier:
.035” #1936 .039” #1939 .043” #1944
.037” #1937 .041” #1942 .045” #1946
Primary Metering Rods (pairs) for 1975 & later (NOTE: These are “M”-series rods with .036” power tips!):
.048” #1941 .052” #1945
.050” #1943 .054” #1947
Secondary Metering Rods (pairs) for all years:
CC #1950 CK #1952 CL #1954
CE #1951 AY #1953
Primary Metering Jets (pairs) for all years:
.068” #1968 .072” #1972 .076” #1976
.069” #1969 .073” #1973 .077” #1977
.070” #1970 .074” #1974
.071” #1971 .075” #1975
You can also order a very few original GM parts from GMPartsDirect on the Internet. The following is a complete listing of the GM part numbers for all available Q-Jet jetting components. These parts are being discontinued quickly, so some parts may no longer be available. Any parts available from GMPartsDirect are also available from any GM dealer (if they want to order them for you). You can also get most of these parts from Carbs Unlimited, although they only offer the early primary metering rods:
Primary Metering Rods, ’74 & earlier:
All rods have been discontinued
Primary Metering Rods, ’75 and later:
There are no post-’75 passenger car (.026” power tip) available from GM at this time.
Secondary Rods:
Code P/N Dia of Tip Tip Length (Short, Medium, Long)
AX 7033549 0.0400 S
BG 7034822 0.0400 M
AH 7033812 0.0530 M
AN 7034320 0.0700 S
Primary Metering Jets:
7031969
7031970
7031971
7031973
7031974
7031975
7031978
The last good aftermarket source for Q-Jet parts is from Carbs Unlimited. They carry a full line of jets and the rods for the early applications. They also carry parts such as choke pulloffs, inlet fittings, springs, and linkages. Go to their website to see a full line of parts.
Questions, Comments & Technical Assistance
If you have questions or comments regarding this article, or if you notice any errors that need to be corrected (which is quite possible since I’m writing this from memory…), please feel free to drop me an e-mail. Also, if you need any technical assistance or advice regarding this process, or other maintenance issues, feel free to contact me:
V8FastCars@msn.com