FEATURE ARTICLE from Hemmings Muscle Machines

The Inner Workings of the Quadrajet

Hemmings Muscle Machines - MAY 1, 2007 - BY RAY BOHACZ

Part 1

Every time someone writes to my Ask Ray column and refers to the Rochester Quadrajet as a Q-Junk, it bothers me. Throughout the years, I have been a champion for this often-maligned fuel metering device. Though the Q-Jet was intended to be a production carburetor, Rochester engineers made sure their creation would suit the needs of the performance community. But, a caveat was attached: You needed to understand how it worked. With that in mind, HMM is providing a primer on the Q-Jet, which will show you how to professionally tune, adjust and maintain this wonderful carburetor.

When working with any carburetor, often a drivability issue is not the result of an internal problem, but external adjustments, cleanliness or other aspects of the engine. Improper ignition system function, rate of advance, incorrect base timing, fuel dilution of the engine oil and vacuum leaks are all areas that impact a carburetor's function and often are blamed on the air and fuel mixer. Thus, before taking a Q-Jet apart, you need to go through some external checks. If the poor performance condition is not alleviated by these steps, then the carburetor will need major attention. Keep in mind there is no reason why a properly tuned and adjusted Q-Jet cannot run perfectly on a muscle car. So, if your car is doing something wrong, do not blame the technology, but instead question the skill of the person who set up the fuel system. There is no need to accept anything but electronic fuel injection-like drivability from a carburetor.

We went to Crossflags Corvette, owned by Bill Lauricella and Lou Ventriglia Sr. to use a customer's 1972 Corvette as our subject. The 350 V-8 still has its factory Q-Jet, but it was in need of some attention. Follow along as HMM shows you how to make your muscle machine run like a 2007 model.

The Q-Jet first appeared for the 1966 model year and replaced the Rochester 4G and 4GC carburetors. The basic design was produced until 1988. From 1981, until the end of production, the Q-Jet was electrified with a mixture control solenoid, which was part of GM's Computer Command Control engine management system. This photograph is typical of what a Quadrajet will look like on an older vehicle.

Depending on the GM division, a Quadrajet may have a front or side-mounted fuel filter/inlet. Early models used a bronze filter that was later replaced with a paper element. The spring behind the filter is used to keep the element against the incoming fuel and force it through the media. No spring, no filtration. The little valve in the paper filter is a roll over device that stops fuel flow in case of an accident. It should not be removed as many enthusiasts do. The Quadrajet is a progressive four-barrel design with an air valve actuated secondary circuit. The idle mixture screws are located in the base plate for superior idle quality.

The Quadrajet is considered a three-piece design: air horn, float bowl and throttle body. The part number is cast into the secondary side of the float bowl and needs to be referenced for rebuilt kits and high-performance tuning parts.

Some applications from 1967 to the late 1970s used a hot idle compensator that was located in the rear of the float bowl (our feature car had the place in the casting, but was not equipped with one). It was used to compensate for an overly rich mixture from excessive fuel vaporization in high-heat conditions. A bimetallic spring that responded to temperature would then open a port and introduce fresh air into the idle circuit to lean the air/fuel ratio.

The accelerator pump is actuated by a lever that attaches to the throttle linkage. To remove the air horn, you will need to remove the roll pin that holds the accelerator pump. Make sure a gap is left to allow prying the pin back for reinstallation. Many Quadrajets have had the air horn casting broken trying to reinstall the accelerator pump arm. All pre-electronic control versions held the air horn to the float bowl assembly with nine screws. The feedback designs used 13 fasteners.

With the choke spring removed, the secondary lock-out-lever is in view. Its function is to keep the secondary barrels from opening if any choke spring tension is present. A misadjusted choke spring or a gummed-up lock lever will mean our muscle machine will run on only two barrels at full throttle.

Early versions used a drill bit index to adjust the choke pull-off opening. Later versions employed an angle gauge specification. Some carbs had both a primary and secondary pull off. Note the linkage connecting the pull-off to the secondary air valve. A swollen diaphragm in the pull-off can either delay or limit the opening of the secondary air valve. For proper cold drivability, it is essential that fast idle speed and pull-off opening are set to specifications. Quadrajets are very sensitive to this adjustment. When out of calibration, either a cold stumble (spit though the intake) or stall will occur if the pull-off opens the choke too much.

The secondary metering rods are attached to a pivot that is operated by a cam connected to the air valve. When the throttle is floored the butterflies open in the throttle plate and allow air to rush by in lockstep with the engine piston velocity. The amount of air then controls how quickly the secondary air valve is blown open. The metering rods lift and allow fuel flow from the secondary jets. This is in contrast to a Holley that uses a vacuum diaphragm to operate the secondary butterflies. The metering rods are changed to tune the wide-open throttle mixture. Thinner rods will allow more fuel to flow through the secondary circuit. When turning a Quadrajet, you also can alter the hanger ratio and the cam profile that lifts the rods for faster response. If the cfm rating of the carburetor is too large for the engine, the secondary will never function since the depression created will not be strong enough to blow the air valve open.

The secondary opening rate is tuned through a spring tension adjustment. There is a small Allen-head set screw under the air horn and a screw that is used to wind the spring. Remember, the choke pull-off also has control over the opening rate. Many tuners use a choke pull-off from a 1970-1971 454 Corvette, since it was designed to allow the secondary to open the quickest. Try to avoid an auto parts store "one-size-fits-all" choke pull-off if you want the best performance.

Want to increase a 650cfm Quadrajet to a 750cfm? Simply grind some material from the secondary stop lever. If your secondary air valve does not open perpendicular to the air horn, then the airflow is limited. Grind this tang to make sure the air valve fully opens. There were no internal differences between a 650cfm and a 750cfm Quadrajet other than the factory setting that allows the air valve to open all the way on the 750 model.

With the air horn removed, the secondary emulsion tubes and fuel discharge port are visible. The fuel discharge is the two large, metal pipes that are pointed toward the air valve. When the air valve is open, the fuel will dump from these pipes into the secondary venturi due to the low pressure created by the rushing air.

All Quadrajets are a spread-bore design. This means the primary side has not only smaller throttle plates, but is closer to the center than the secondary. Note how the casting bows as it approaches the secondary butterflies. To the right, is the fast idle screw.

One attribute of a Quadrajet is a three-ring booster venture cluster in the primary. This provides excellent fuel atomization and drivability over the common two-booster design. The ring-within-a-ring design is the trademark of a Quadrajet. The round cavity to the top right would house an aneroid valve for altitude compensation. As seen here, the primary jets are threaded-in, but the secondary jets are sleeves pressed into cast orifices. For this reason, many tuners find favor with Holley's four-jet design since it is easier to tune.

A common problem with all Quadrajets is throttle-plate bushing wear, which leads to an air leak around the throttle shaft, causing an unstable idle; this can be repaired by a specialty shop. Another malady is a warped air horn. When his occurs, the gasket does not seal against the float bowl and fuel is pulled from the intermediate or main metering circuit at idle. If you have a Quadrajet that is very rich at idle, no matter what you do, check for a warped air horn. If a replacement is not offered, a machine shop can mill the part to correct the problem.

To the side of the accelerator pump bore is a metal plate and below that is the discharge check ball. It is located under a screw in the bottom of the float bowl. If either one of these is missing, the engine will have a tip-in stumble since the accelerator pump will not function properly.

The accelerator pump uses two springs: one on the assembly and another underneath the plunger cup in the bore. The pump is designed to deliver fuel over time, not all at once. That is why a specially calibrated spring is employed.

It's common to find that the secondary-well feed tubes have fallen out of the air horn. When this happens, a wide-open-throttle (WOT) bog will occur due to a delay in delivering fuel to the secondary barrels. This is often incorrectly diagnosed as an air valve that is opening too quickly. However, it is a lack of fuel. You may need to stake the tubes or spread them out slightly to recreate a tight fit.

The float level is extremely critical and an often overlooked part of tuning. The float level is measured from the rear of the float to the top of the float bowl casting. To adjust the float, it needs to be removed to gently bend the lever. During the 1970s, many Quadrajets had porous floats that would absorb fuel and create an overly rich mixture. If yours looks to have deteriorated or seems heavy, replace it.

Unlike a Holley, which uses a power valve, the Quadrajet has primary metering rods and a power piston, which is sometimes referred to as an adjustable part throttle (APT). The piston travel is altered with a threaded "double-D" screw that is barely seen here (A). Note the aneroid cavity plug (B) and the float baffle (C). The baffle is used to control fuel movement and stabilize the float and should be retained on all street applications.

The single largest Quadrajet problem is hard starting when cold. Often, the main well plugs leak and allow the fuel bowl to empty when the engine is shut off. When built, this passage is drilled to size the main well and then sealed with a lead plug. Covering the bottom of the plugs with a little quick-set epoxy is usually all that is required to correct the problem. If the carburetor is going to be used on a high-horsepower engine, it may be necessary to drill these passages larger and then seal them with a steel plug and epoxy. Even if your Quadrajet gives you no problems, when it is apart, put a dab of epoxy on the plugs.

If you are looking to have your Quadrajet rebuilt and modified to wake up your GM muscle car, we suggest you speak to Bob Wise of RaceKrafters in Lancaster, Pennsylvania. Wise is recognized as one of the country's top Quadrajet tuners. His knowledge has been gathered first hand. For a number of years after leaving the Marines, he was a carburetor and EFI calibration technician at the GM Proving Ground in Mesa, Arizona. He now works with brother, Craig, building carburetors, EFI systems and race engines. You can mail your carburetor to their shop. All carburetors are dyno-tested before being returned to the customer.


RaceKrafters Auto Machine
1140 Dillerville Road
Lancaster, Pennsylvania 17601

Crossflags Corvettes
11 Route 46 West
Mt. Olive, New Jersey 07828

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This article originally appeared in the MAY 1, 2007 issue of Hemmings Muscle Machines.
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