Rochester Quadrajet M4M Series

The M4M series (M4MC, M4ME, M4MED and M4MEF) Quadrajet carburetor, Figs. 2 through 5, is a four barrel, two stage unit consisting of three major assemblies: The air horn, float bowl and throttle body.
A single float chamber supplies the fuel to the four carburetor bores. A float, float needle and float seat are used to control the fuel level within the float chamber. A vacuum operated power piston and metering rods control the air/fuel metering in the primary throttle bores. The tapered metering rods are attached to the power valve piston assembly, moving in fixed metering jets to provide the various fuel flow requirements of the engine. A factory adjusted part throttle screw is used on all models to properly position the metering rods in the metering jets. The M4MEF model, in addition to the part throttle screw mentioned previously, also contains a rich stop adjusting bushing to position the enrichment portion of the metering rods in the jets. This bushing enables an adjustable wide open throttle mixture control capability. During wide open throttle operation, the air/fuel mixture in the secondary bores is controlled by air valves and tapered metering rods operating in fixed jets. On M4MEF models, the factory adjusted secondary well air bleed adjusting screw provides additional control of the air/fuel mixture during wide open throttle modes.
The accelerator pump system on all models uses a conventional throttle actuated pump plunger operating in a pump well, to provide additional fuel as needed. In addition, M4MED models are equipped with float bowl mounted dual capacity pump valve and solenoid assemblies. These assemblies provide a smoother transition from idle to part throttle operation during cold engine driving modes.
All carburetors use either electrically or hot air heated choke coils. The heated coils provide choke valve closing force for cold starts, as well as correct opening timing during warm-up. Vacuum break assemblies control initial choke valve opening during starting and warm-up periods.
Curb idle speeds are controlled, depending on engine and application, by either an idle speed adjusting screw or an idle speed solenoid. On vehicles without air conditioning, the idle speed solenoid provides desired idle speed, while preventing dieseling when the ignition switch is turned off. On vehicles with air conditioning, the solenoid maintains smooth idle during compressor clutch engagement. Vehicles without an idle speed solenoid are equipped with a throttle kicker assembly. The vacuum operated throttle kicker holds the throttle open during deceleration, thereby reducing exhaust emissions. On some engines, it is also used to increase idle speed depending on accessory load.