Carburetor - ROCHESTER M4M Series 4-Bbl: Description

The M4M-series carburetors are all 4-barrel, two-stage designs. The primary side has a triple venturi system. The secondary side is composed of 2 large bores using the air valve principle (fuel is metered in direct proportion to amount of air passing through secondary bores). M4MC models use a hot air choke; all others use an electric choke.

M4MEF models (GM only) have adjustable wide open throttle mixture control. During wide open throttle conditions a factory-set secondary air bleed adjusting screw provides more precise control of the air/fuel mixture.

A dual capacity accelerator pump valve assembly, mounted in the float bowl, is used on M4MED models (GM only). A dual capacity pump solenoid is used. When the engine is cold, more fuel is necessary to ensure a smooth transition from idle to part throttle. When the engine is warm, less fuel is needed. A coolant temperature sensor energizes a pump solenoid when coolant temperature exceeds 170°F (77°C). This solenoid opens the dual capacity pump valve, reducing pump capacity by about one-half. The GM dual capacity accelerator pump solenoid is powered up via a PINK/BLACK ignition feed whenever the ignition key is turned on. A BLACK/GRAY wire connects the other side of the solenoid to the coolant temperature sensor. See above for coolant sensor operation. The solenoid's electrical connector looks identical to a standard GM mixture control solenoid connector.