The Quadrajet is a 4 barrel carburetor made by the Rochester Products Division of GM that was first used on the Chevrolet 396 Mark IV engine in 1965 then widely used on General Motors motor vehicles until 1990. Its last application was on the Oldsmobile 307 V8 engine, which was last used in the Cadillac Brougham and full size station wagons made by Chevrolet, Pontiac, Oldsmobile, and Buick. Not widely known, the Q-jet was used briefly by Ford on the 1971 429 Cobra Jet and was also used by Mopar on their 360 truck engines in the '80s.
The Quadrajet has been described as one of the best carburetors made for passenger vehicles, offering the best compromise of fuel economy, performance, driveability, and emissions control.
 Basic design
The Quadrajet is a "spread bore" carburetor; the primary venturis are much smaller than the secondary venturis, and the center to center spacing of the throttle bores are different, primary to secondary. By comparison, a "squarebore" carburetor may have primary and secondary venturis of similar if not exactly the same size and the center to center spacing of the primaries and secondaries are the same, thus the name "squarebore".
Most Quadrajets made prior to 1976 were capable of 750 cubic feet per minute (CFM) maximum air flow measured at a 1.5"/Hg pressure drop, but some were manufactured as 800 CFM (and larger, in isolated cases like the '71 Pontiac H.O., more below). The increase in airflow was due to a larger primary venturi. This was first done in 1971 by the Pontiac and Buick division of GM for use on their 455 cid/high performance engines.
In 1971 Pontiac had designed a special Q-jet that had the most flow of any production Q-jet, but because the modification decreased the vacuum signal on the primary side too much it was discontinued, so it was a "one year wonder" that fetches outrageous prices from restorers today. Many more 800 cfm Q-jets were installed from 1976-up. One way to locate a large casting carb is to source one from an '80-up light truck. Even the Chevy/GMC 4.3L V6 trucks used the large casting 800 cfm Q-jet!
 Power enrichment
Most Quadrajets use a vacuum operated "power piston" (or "PP") to move the primary metering rods to control the air/fuel ratio, allowing the mixture to be leaner under low load/high vacuum conditions and richer during high load/low vacuum conditions. A less-common, early version uses a linkage attached to the primary throttle shaft to mechanically move the power piston.
There were also two additional Q-jet designs used mainly in 1975 only, that used an auxiliary enrichment system in addition to the primary PP system. There are two types of auxiliary systems: one has an aneroid "bellows" that responds to barometric pressure changes; it used no vacuum or power piston to operate it. The other uses a vacuum operated PP like the primary PP, but it differs by using only one metering rod and one fixed jet to feed both sides of the primaries. These were abandoned soon after they were released; it was found the primary PP system could be tailored to meet the requirements without the added complexity of the auxiliary system. Other changes came on-line in that same era, info on this can be seen at Q-jet variants.
The "E" (Electronic Control Module controlled) series of Quadrajets use a computer controlled mixture control solenoid that responds to electronic signals from the throttle position sensor to the ECM, ideal for precise fuel metering and allowing additional fuel under load. The solenoid-controlled metering rods allow the fuel mixture to be very close to optimum, then the solenoid is pulse width modulated at about 6 Hz to fine-tune the air fuel ratio under closed loop conditions. The electronic versions have a throttle position sensor that is mounted inside the carburetor body, actuated by the accelerator pump lever.
Quadrajet carburetors have mechanical secondary throttle plates operated by a progressive linkage (primaries open before secondaries) but use "on-demand" air valve plates above the secondary throttle plates. The secondary fuel metering rods are lifted by a cam-actuated hangar connected to the air valve shaft. As the airflow increases through the secondary bores, the air valves are pushed down (open), rotating a cam that lifts a hangar that holds the secondary metering rods. The secondary rods are tapered in a similar fashion to the primary metering rods, effectively increasing the size of the fuel metering holes as the rods are lifted, thus delivering more fuel. Therefore, the position of the air valve controls both fuel and air flow through the secondary venturis, even if the secondary throttle plates are fully opened. The end result is that the Quadrajet acts like a "vacuum secondary" carburetor and only delivers more air/fuel if it is needed.
Significant positive features of the Quadrajet were:
- Economy. Unlike most other 4-barrel carburetors, the Quadrajet has a drastically different sized primary and secondary bores. The much smaller primaries act as a small two-barrel carburetor until you press the throttle enough to start to open the secondaries. The small primaries allow the primary throttle plates to be opened wider, and thus making the carburetor more efficient than a large two barrel, or square bore four-barrel.
- Drivability. The small primaries also create better throttle response at part throttle application. The Quadrajet had a centrally located float that gave it excellent fuel control resulting in excellent street manners.
- Off Road. The Quadrajet’s centrally located float is highly resistant to level changes caused by cornering or acceleration.
Significant negative features of the Quadrajet were:
- Leaking fuel bowl. As in nearly all carburetors, the Quadrajet float bowl has pressed-in plugs used to seal holes left after drilling fuel passages during the manufacturing of the carburetor. These plugs in the Q-jet sometimes leaked fuel causing:
- cold engine being hard to start
- erratic idling
- poor fuel mileage
- excessive emissions
- Fortunately, leaking well plugs is primarily a problem only on the first few years of production when pressed in cup plugs were used instead of the later (and better) spun-in aluminum plugs that were swaged in place. Many Quadrajets have their fuel bowl plugs sealed with epoxy when rebuilt to prevent leaks. This may be good for a short time but epoxy will not stand up to the extreme conditions (heat and fuel contact) encountered for long.
- The small float bowl can result in fuel starvation in extreme high-performance situations, but can usually be traced to a fuel delivery problem to the carburetor, such as a worn fuel pump, pump push rod, or camshaft eccentric. In normal driving and even off road performance use it provides excellent control of the fuel level.
- The fuel inlet/fuel filter housing threads tend to be very fragile. It is common for the fuel inlet threads in the main casting to strip. There are several "fixes" available in the aftermarket: New oversize self-tapping fuel filter housings; new fuel filter housings that seal with O-rings; and Heli-Coil rethreading kits.
- Almost all Quadrajets today have some amount of warpage of the castings. The root cause of this warpage is often over-tightening the front two carburetor mounting bolts, often in combination with a base gasket that doesn't have hard nylon inserts for the bolt holes. Without the inserts the base gasket can compress, warping the castings.
- The steel primary throttle shaft will tend to wear the aluminum casting material. This results in an air leak and in extreme cases could cause the primary throttle blades to not close properly. This can result in poor idle quality and unfiltered air to be pulled into the engine. The aftermarket has responded; several vendors are supplying bushing repair kits for the Q-jet throttle body.
 Q-jet variants
A major change to the Quadrajet was implemented for the 1975 model year. These newer carburetors are considered "Modified Quadrajets" or "Mod Quads". In addition to the casting revisions that resulted in a somewhat physically larger carburetor, the primary metering rod length is different (shorter) than those from '74 and older Q-jets. They were also equipped with a self-contained choke mechanism that no longer relied on a choke coil mounted on the intake manifold above the heat crossover port. Also the number "1" was added to the beginning of their identification numbers. Both style carbs (divorced choke and integral choke) were produced concurrently.
Quadrajet carburetors were also built under contract by Carter. This seems to have happened at times when Rochester's facility could not keep up with demand. Carter-built Quadrajets will have the name "Carter" cast into them, but are functionally identical to the Rochester-built equivalent. The "newest" Q-jets to be made were built for, and sold by Edelbrock. There were several versions made, for both stock replacement and "performance" applications. One version was specifically intended as a replacement for Carter Thermoquad carburetors. The Edelbrock Q-jet carbs have been discontinued, although at this time Edelbrock still supplies some tuning and replacement parts. Return to top
 So, all Q-jets are the same, right?
Not hardly! These are a few differences:
Generally, there were the ‘small’ and ‘large’ casting carb bodies. These were 750 and 800 cfm, nominal- other cfm requirements were met by tailoring the air valve and/or the secondary throttle stop. Then there was the rare (one year-‘71) Pontiac HO carb that used a smaller booster to get even more flow than the “normal” large casting (800 cfm) carb.
There were straight or 90° fuel inlets, w/short and long filter housings. There were at least 4 different choke arrangements. There were 5 different idle mixture screws. 4 different float pivot pins, 5 different float bowl inserts. There were both Torx and straight blade screw heads used to secure the air horn.
There were two different needle and seat arrangements regarding the type, and at least 3 different floats made from 2 different materials (nitrophyl and brass).
Later carbs (~’76-up) have an adjustable part throttle feature (aka "APT"). Some have idle air bypass, others don’t. Most had only two primary rods, some had three- one hooked to an aneroid. Sort of like 2 power pistons.
Various carbs had different size and shape bowl vents in various locations, some had a hot air compensator. Many had secondary fuel booster outlets (passive accelerator pump) that were located above the air valve, others below (below better for high performance). Some had a primary arrangement similar to the secondary system just described. Some secondary air valves were slotted, others not.
Accelerator pump pistons varied by length and spring rates (at least 6 different), inlet seats were different sizes (at least 6 different sizes available, 0.095” to 0.149” [0.149” is aftermarket only]), power piston spring rates varied (10 different OEM springs, others available through the aftermarket), obviously jets (at least 13 different OEM sizes, from #64 to #78), primary metering rods (15 different '65-'67 rods and 18 different ‘68-up rods. Sizes range from 0.033” to ~0.060”), secondary metering rods (93 to select from), secondary hangars (20 different p/n’s) varied between carb applications. Throttle linkages varied greatly- some were made for manual trans apps, these can be used w/a TH400 if the kickdown switch is relocated (or is at the throttle pedal), some were only for a TH400 apps w/carb-mounted kickdown switch, some were used w/the TH350 (has the ability to "pull" a detent cable), some for the Powerglide trans. Some have cruise control studs, some not. Early carbs used throttle linkage, later used a cable.
There were at least 20 different front choke pull offs, at least 10 different rear choke pull offs, carbs with front only or both front AND rear choke pull offs, some used no pull offs at all, instead relying on a piston in the fuel bowl to damp the secondary opening rate. Vacuum port size, number and locations were all over the place.
There are two different length primary rods, there are stepped and tapered rods. The bottom line to this is there are [b]millions[/b] of different combos possible for the '68-up carb alone! And this doesn't even consider all the different linkages, choke pull-offs, the two different CFM ratings, etc., ad infinitum.
 Choosing a carb
You need to pay attention to a few important things when choosing a carb to rebuild/use on your engine:
- Fuel inlet (straight or 90º), so it doesn't interfere with any of the accessories or water neck, etc.
- Choke type. Most prefer an electric. A hot air type choke can be converted to electric easily. A divorced choke Q-jet is able to be converted to electric, however the kit is rather expensive.
- Whenever possible, get the factory cable bracket. Even if you're using an aftermarket intake, it might come in handy for making up your own bracket. And regardless if using an aftermarket intake or not, the bracket can be used to measure the correct cable/carb relationship for setting up whatever bracket that's used.
- Whenever possible, make note of the engine displacement/engine VIN code. These carbs were used on 4.3L V6 through 7.4L V8 engines, so knowing what it came from gives a leg up on what may potentially need changed.
- Throttle arm- whether or not it can accommodate the trans you're using. This is important if using a TH350 that needs a "below-the-shaft" hook-up point to pull the detent (kick down) cable, or when using a GM OD automatic that uses a TV (throttle valve) cable (below, right). Many manual transmission-equipped vehicles are missing the lower part of the throttle arm (below, left); this is where the detent/TV cable ordinarily attaches.
Beyond that, you basically just work with what you have. The Q-jet is very versatile and will work fine on any number of engines.
1976-up carbs are often preferred due to less wear than the older carbs, often an electric choke will be present, and the float and needle and seat assembly setup is better. Also because there were improvements made throughout the run of Q-jets, newer is better. The '76-up carbs will also have APT (adjustable part throttle, a way to fine tune the carb).
Cars (not necessarily trucks/vans) from '81-up are going to have feedback carbs. No good for anything other than computer use (CCC), or possibly as an all-out race carb where it is set up/jetted to run without primary metering rods.
Interestingly, one of the best/easiest way to find a good carb to use is to look for one off of a truck or van. Up until '86, these truck carbs were still non feedback (not an "electronic" carb) in some applications and they were nearly all 800 cfm units- even those found on 4.3L V6 engines! These engines can also have the large coil-in-cap, non-feedback HEI distributors.
It's also a good idea to acquaint yourself with the "dual capacity" accelerator pump Q-jets, in case you run across one. They can make a good performance carb.
 Power Piston Springs
The power piston (PP) spring is seldom mentioned when Q-Jet tuning is being discussed, but it need to be addressed just like if it were a Holley. The same principals apply when tuning for low vacuum cams, etc. The power piston spring allows the power piston to behave like a Holley's power valve. That's to say the power enrichening system is vacuum controlled.
At high vacuum, the vacuum exceeds the PP spring rate and the primary rods are pulled down into the main jets, leaning the fuel/air mixture. Conversely, when vacuum drops like when under a load or the accelerator pedal is whacked WFO, the PP spring rate exceeds the vacuum, which lifts the rods up to their smaller diameter "rich" position.
If a “medium” PP spring = a Holley 6.5 in/Hg, a "soft" PP spring (allows enrichening to come in at a lower vacuum) would be like a 3.5 Holley PV; a “stiff” PP spring = a Holley PV of, say, 8.5 in/Hg. Longer duration cams will “like” a softer PP spring.
Below is an image showing a variety of different springs, another shot shows three different types of power piston. The springs vary in length, coil diameter and wire diameter (0.012"- 0.020" in my collection).
Edelbrock has different PP springs in a "Race Calibration Kit" that contains a selection of main jets and rods, secondary rods, hangars, PP springs, high flow needle and seat and an accelerator pump. Gessler and Ruggles both carry a selection of PP springs as well as many other tuning and repair parts.
GM p/n 7029922 is a rather weak spring that was OEM for HO Pontiac engines from the early '70's. It delays enrichment until vacuum drops to 3 in/Hg. This spring will work with durations >/= 230 degrees @ 0.050”. Another GM p/n is 7037305 for a PP spring that is set for 6 in/Hg, and would work well w/a high vacuum, smooth idling and/or wide LSA-type cam. Thing is, unless the parts counter guy is cool w/you, they come in packs of 10 springs.
 Secondary metering hangars and rods
 Intake manifolds
 Tuning and repair parts
- Cliff's High Performance
- Carbs Unlimited
- The Carburetor Doctor
- SMI Sean Murphy Induction
- Service manual
 See also
- Crankshaft Coalition wiki Carburetor articles
- Hotrodders Knowledge Base Carburetor articles - contains links to various useful carburetor websites and threads.