Quadrajet

Design

The Quadrajet is a "spread bore" carburetor; the primary venturis are much smaller than the secondary venturis. By comparison, a "square bore" carburetor has primary and secondary venturis of similar size. Most Quadrajets were capable of 750 cu ft (21,000 l)/min (cfm) maximum, but some rare Buick and Pontiac models were capable of 800 cu ft (23,000 l)/min for use on high performance engines, and most 1984-1987 pickup trucks were also equipped with the 800-cfm carb. Most Quadrajets use a vacuum operated piston to move the primary metering rods to control the air-fuel ratio, allowing the mixture to be lean under low load conditions and rich during high load conditions. A less-common version uses a linkage driven off the primary throttle shaft to mechanically move the power piston. "E" (electronic control module controlled) series of Quadrajets use a computer controlled mixture control solenoid that responds to electronic signals from the throttle position and oxygen sensors via the computer, 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; the primaries open before the secondaries, and use on-demand air valve plates above the secondary throttle plates. The air valves are connected by a cam and linkage to the secondary fuel metering rods. As the airflow increases through the secondary bores, the air valves are pushed down, rotating a cam that lifts 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 and delivering more fuel. Therefore, the position of the air valve will control both fuel and air flow through the secondary venturis, even if the secondary throttle plates are fully opened. Thus the Quadrajet acts like a vacuum-secondary carburetor and only delivers more fuel as it is needed.

Advantages

Significant positive features of the Quadrajet were:

1. Economy. Unlike most other four-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 the accelerator is pressed far 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.
2. 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 form the triple Venturi , boosters.
3. Off Road. The Quadrajet’s centrally located float is highly resistant to level changes caused by cornering or acceleration.

The Quadrajet carburetor was actually a derivative of a variable venturi carburetor called the DOVE (diaphragm operated variable entrance) which was developed in the 1961-63 time frame at Rochester Products. Testing at the GM test facility in Arizona uncovered a hot engine percolation problem which resulted in hot start difficulties because of flooded engines. Production of the DOVE, which was underway in 1963 when the hot start problem was identified, was suspended and a crash project was initiated to fix the problem. Simultaneously a second crash project was initiated to develop a modified DOVE which became the Quadrajet. Prototype Quadrajet carburetors were in test at Rochester Products by the Fall of 1963. The DOVE hot-start problem was corrected but not in a timely enough manner; the production DOVEs were destroyed and the Quadrajet took its place.

Drawbacks

Significant negative features of the Quadrajet were:

1. Its leaky fuel bowl. As in nearly all carburetors, the Quadrajet's bowl had 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 difficult cold-engine starting, erratic idling, poor fuel mileage, and excessive emissions. Many Quadrajets have their fuel bowl plugs sealed with epoxy when rebuilt to prevent leaks.
2. The very small float bowl/fuel chamber can result in fuel starvation in extreme high-performance situations, but can always be traced to a fuel delivery problem to the carburetor, such as a weak fuel pump or a worn/rounded camshaft eccentric that drives the fuel pump lever.
3. The fuel inlet/fuel filter housing threads tend to be very fragile. When care is not taken to align the insert, it is possible for the fuel inlet to cross-thread and to strip when tightened in the main housing. There are several "fixes" available in the aftermarket: New, oversized, self-tapping fuel filter inserts; new fuel filter inserts that seal with O-rings; and Heli-Coil re-threading kits. In nearly all cases, the carburetor will require dis-assembly and internal cleaning of the aluminum thread residue, especially up to and including the needle and seat, (needle valve), to prevent flooding.
4. Almost all Quadrajets today have some amount of warpage of the castings although less pronounced in the so-called "mod Quad" versions after 1974 which were a bit heavier and better designed to resist warping. The root cause of this warpage is over-tightening the front two carburetor mounting bolts, often in combination with a base gasket that lacks hard nylon inserts for the bolt holes.
5. Over much use, the steel primary throttle shaft will tend to wear the aluminum casting material in the throttle body. This results in a minor air leak and in extreme cases, can cause the primary throttle blades to not close properly. This results in poor idle quality. The aftermarket has responded; several vendors are supplying repair kits for the carburetor body, generally in the form of teflon bushings.
6. The carburetors have been billed as difficult to tune and unreliable by some, however, many times this is simply attributed to the lack of knowledge of the unit itself. Consequently, many Quadrajets were replaced with other brands of carburetors in an often ill-fated attempt to rectify a problem that was not caused by the carburetor. When properly tuned, the Quadrajet is both an efficient and effective performing carburetor.

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 result in a physically larger carburetor, the primary metering rod length is different from '74 and older Q-Jets. They were also equipped with a self-contained choke mechanism that no longer relied on an intake manifold mounted choke, and a number "1" was added to the beginning of their identification numbers.. The digits in a Quadrajet model type denote its features. For example, the "E" at the end of a later Q-jet model denoted that it had an electric choke, the "C" denoted a hot air coil style choke element. Original Delco service kits were once sold through both GM dealers and Delco distributors and were called "Power Kits". These have long been discontinued, although there are several aftermarket sources that still supply parts for these carburetors.

Choke Variants

The choke provision for the Quadrajet was initially in the form of an intake-mounted, heat sensitive spring, (divorced choke), often referred to as a heat riser. The spring connected to a rod that actuated the choke mechanism on the passenger's side of the carburetor, and relied on intake manifold's temperature. Later models, second generation Quadrajets, (1974-onward), were designed with a self-contained choke housing that held the heat sensitive spring and directly operated the choke mechanism. This system relied on hot air drawn from a small heat exchanger in the intake manifold -- and later models, (generally 1978-onward), relied on the vehicle's 12 volt system to power a heating element and spring as the engine's temperature increased.

Quadrajet carburetors were also built under contract by Carter. This was due to the fact that Rochester could not keep up with the demand for carburetors at various points in their production. Carter-built Quadrajets will have the name "Carter" cast into them, but are functionally identical to the Rochester-built equivalent. The "newest" Q-Jets 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-Jets have been discontinued, although at this time Edelbrock still supplies some replacement parts.