Carburetor: Description and Operation

Quadrajet Carburetor:

CARBURETOR
The Rochester Quadrajet is a four barrel two stage carburetor with three main assemblies and six operating systems. The main assemblies consist of the air horn, float bowl, and throttle body. The operating systems consist of the Float system, Idle system, Main metering system, Power system, Accelerator pump system, and Choke system.
FLOAT SYSTEM
The float system maintains an adequate supply of fuel in the fuel bowl for the fuel delivery systems. The single float chamber supplies fuel to all the 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.
IDLE SYSTEM
Each primary carburetor bore has a separate and independent idle system to provide the correct air/fuel mixture to the engine during idle and off idle operation. The idle system is required because air flow through the carburetor venturi is insufficient to obtain correct metering from the main discharge nozzles.
MAIN METERING SYSTEM
The main metering system supplies fuel to the engine during part to full throttle operation. The system mixes air and fuel in correct ratios during this range through plain tube nozzles and the venturi principle. The multiple venturi in each bore produces excellent fuel metering control due to its sensitivity to air flow.
POWER SYSTEM
The power system provides extra fuel enrichment to meet power requirements under heavy engine loads and high speed operation. The richer mixtures are supplied through the main metering systems in the primary and secondary sides of the carburetor.
ACCELERATOR PUMP SYSTEM
During quick acceleration with rapid throttle opening, air flow through the carburetor bores and intake manifold changes almost instantly. However, the fuel, which is heavier tends to lag behind causing a momentary leanness. To prevent this the accelerator pump system is used to provide the extra fuel necessary for smooth acceleration.
CHOKE SYSTEM
The choke valve mounted in the air horn above the primary carburetor venturi provides the correct air/fuel mixture enrichment for cold engine starting. The choke valve also assists engine running during the warm up period. The choke system uses an electric heat coil mounted in the choke housing, and this coil in combination with the choke valve and throttle position controls choke operation.
CARBURETOR IDENTIFICATION
The Rochester Quadrajet carburetor, comes in several versions, based upon the production number stamped vertically on the identification plate, found on the drivers side rear corner of the carburetor.