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GENERAL DESCRIPTION
The Rochester Quadrajet is a four barrel, two stage carburetor of down draft design. It has been proven to be dependable and a versatile performer as witnessed by its broad application and usage since its first introduction in 1965. Modern demands for greater fuel economy and improved emission control have resulted in significant refinement of the original design. The Rochester Quadrajet has evolved to the point of being an efficient and sophisticated fuel control device that is right for the times and yet maintains the serviceability that is so important to those responsible for vehicle performance and customer satisfaction.
The Quadrajet carburetor has two distinct and separate design stages. Each stage operates to provide a blend of economical operation and responsive engine performance.
The primary side of the carburetor (fuel inlet side) has two small bores. Each bore contains a triple venturi arrangement equipped with plain tube nozzles. The triple venturi provides excellent fuel atomization and delivery in the off idle and part throttle ranges of engine operation.
Fuel metering is controlled through the usage of tapered primary metering rods that are suspended within finely machined metering jets. The metering rods are moved vertically by engine vacuum control. This is accomplished by the use of a spring-loaded piston and rod hanger assembly and results in the control of fuel allowed to pass through the metering jets and to the engine during low speed and moderate speed operation.
Some models use multiple stage enrichment methods that may incorporate additional rods, jets and/or power piston arrangements. There are models that have devices that respond to changes in atmospheric pressure as well. The objective is to provide a high degree of sensitivity to air/fuel mixture control at lower and midrange engine speeds. The primary side of the Quadrajet carburetor has the design features to meet these needs. The individual systems are described in detail later in sections of this text.
The secondary side of the Quadrajet has two larger bores. This increased area, when combined with that of the primary side of the carburetor, provides an air delivery capacity that can satisfy a broad range of engine operating conditions.
An air valve, positioned above the secondary bores, controls the fuel delivery for high demand conditions. This secondary air valve is connected mechanically to a pair of secondary metering rods that are tapered and are suspended within a pair of fuel metering orifices. The fuel passing through these secondary orifices is directed to a set of delivery nozzles located at the top of each secondary bore and below the air valve. The fuel flow through the nozzles is thereby controlled in direct proportion to air flowing through the secondary bores.
There are two basic carburetor model designs:
First released in 1965, the 4MV is an automatic choke model designed for use with a manifold mounted thermostatic choke coil. The 4MC model also is an automatic choke carburetor but with the choke thermostatic coil located in a choke housing mounted on the side of the float bowl. Except for choke systems, all models have basically the same principles of operation (Figure 1).
First released in 1975, the M4ME models differ from the conventional 4MV-4MC carburetor models in that the “M” prefix designates “modified”, indicating the primary side is revised to accommodate an adjustable metering rod assembly with filler spool, or aneroid-metering rod assembly, where required, and the auxiliary power piston assembly (if used). The aneroid feature is normally used to achieve altitude compensation in air/fuel mixtures (designated Models M4MCA-M4MEA) (Figure 2).
Later “modified” models use separated main wells with an aneroid cavity insert in the float bowl replacing the adjustable metering rod or aneroid-metering rod assembly.
On all models, the fuel chamber is centrally located to avoid problems of fuel spillage causing engine turn cut-out and delayed fuel flow to the carburetor bores. The fuel chamber is relatively small in volume to reduce fuel evaporation during hot engine “shutdown”. A plastic bowl insert is included to reduce the free volume above the fuel level to prevent fuel loss to the main metering jets during critical vehicle maneuvers.
The float system has a single pontoon float and fuel inlet valve for simplification and ease of servicing. An integral fuel filter (or strainer), and check valve (if used), located in the float bowl ahead of the fuel inlet valve, is easily removed for cleaning or replacement.
All models have an Adjustable Part Throttle (A.P.T.) screw located either in the throttle body (since 1968 on 4MV-4MC models) or in the float bowl (“M” modified models) to aid in refinement of fuel mixtures for good emission control.
The thermostatic choke coil assembly on 4MV models is heated by hot exhaust gases that flow through a special passage in the intake manifold. On 4MC-M4MC-and M4MCA models, the thermostatic coil assembly is warmed by exhaust heated air supplied through a tube to the choke housing mounted on the float bowl. M4ME and M4MEA models use an electrically heated thermostatic coil assembly to control choke mixtures after engine start and cold driveaway.
The throttle body is aluminum to reduce overall weight and to improve heat distribution, plus there is a thick throttle body to bowl insulator gasket to keep excessive heat from the float bowl causing fuel percolation.
Some Quadrajet applications use a shim between the throttle body and flange basket. The shim is used to protect the carburetor aluminum throttle body from exhaust gases flowing through the heat cross-over or exhaust gas recirculation passage in the intake manifold.
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