Quadrajet – Slurry Pump EGM – Slurry Pump ELM manufacturer
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 if not exactly the same size. Most Quadrajets were capable of 750 cu ft (21,000 l)/min (cfm) maximum, but some rare Buick models were capable of 800 cu ft (23,000 l)/min for use on high performance engines. 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 sensor, 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 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. The end result is that 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:
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 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 centrally located float is highly resistant to level changes caused by cornering or acceleration.
Drawbacks
A Quadrajet M4ME carburetor with electric choke. The Quadrajet went through several changes in its lifetime.
Significant negative features of the Quadrajet were:
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; (a) a cold engine being hard to start, (b) erratic idling, (c) poor fuel mileage, and (d) excessive emissions. Many Quadrajets have their fuel bowl plugs sealed with epoxy when rebuilt to prevent leaks.
The leaking well plugs were primarily a problem on early Qjets with the pressed-in cap plugs. By the late 1960s, all Qjets were equipped with thick, spun-in plugs that eliminated this problem. Unfortunately, the myth endures.
The very small float bowl/fuel chamber 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 weak fuel pump or worn camshaft lobe. In normal driving and mild performance use it provides excellent control of fuel level.
The fuel inlet/fuel filter housing threads tend to be very fragile. When care is not taken to align the insert, it is common for the fuel inlet to cross-thread or to strip when over-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 rethreading kits. In nearly all cases, the carburetor will require disassembly and internal cleaning of the aluminum thread residue, especially up to and including the needle and seat, (needle valve), to prevent flooding.
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.
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.
The factory plastic fuel-level floats tend to absorb fuel and sink. (This can be cured with a Borg-Warner brass unit.)
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.
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 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.
References
^ Sherman, U. B. “Quadrajet Quickery II”, in Popular Cars, 12/85, p.50.
^ Sherman, p.53.
^ Sherman, p.53.
See also
External links
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Categories: Automotive technology stubs | Automotive technology tradenamesHidden categories: All articles with unsourced statements | Articles with unsourced statements from July 2007 | Articles that may contain original research from September 2007 | All articles that may contain original research
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