Rochester Quadrajet 4 Barrel Carburetor

Fig. 1 Rochester Quadrajet carburetor assembly:

The Rochester Quadrajet carburetor, Fig. 1, is a four barrel two stage carburetor with three main carburetor assemblies; the air horn, float bowl and throttle body. There are six basic operating systems; the float system, idle system, main metering system, power system, pump system and choke system.
The float system maintains an adequate supply of fuel in the fuel bowl for use by various fuel delivery systems. A single float chamber supplies fuel to all carburetor bores. A closed cell rubber float, brass needle seat and plastic tipped float needle with pull clip are used to control fuel level in the float chamber.
Each carburetor bore has a separate and independent idle system to provide the correct air/fuel mixture to the engine during idle and off-speed operations. The idle system is necessary because air flow through the carburetor venturi is not sufficient to obtain efficient metering from the main discharge nozzles.
The main metering system supplies fuel to the engine from off-idle to wide open throttle. The system supplies air and fuel during this range through plain tube nozzles and the venturi principal. The multiple venturi in each bore produces excellent fuel metering control because of its sensitivity to air flow.
The power system provides additional mixture enrichment to meet power requirements under heavy engine loads and high speed operation. The richer mixtures are supplied by the main metering systems in the primary and secondary sides of the carburetor.
Air flow through the carburetor bores and intake manifold changes almost instantly during quick accelerations when the throttle is opened rapidly. The fuel, which is heavier, tends to lag behind, causing a momentary leanness. The accelerator pump system prevents this by providing the additional fuel necessary for smooth acceleration.
The offset choke valve is mounted in the air horn above the primary carburetor venturi. The closed choke valve provides the correct air/fuel mixture enrichment for cold engine starting. The choke valve also aids engine performance during warm up.
An electric choke coil is mounted in the choke housing. This system uses electric current supplied to the choke coil, which, combined with the offset choke valve and throttle position, control choke operation.
Fuel metering is controlled by two special shaped metering rods, operating in the main metering jets, and positioned by a plunger in the oxygen feedback solenoid. The solenoid plunger is controlled by an electrical output signal generated by a computer. The computer, responding to an electrical signal from the oxygen sensor in the exhaust manifold, alternately energizes the solenoid, moving the metering rods to the lean position, and deenergizes the solenoid, moving the metering rods to the rich position.