Carburetor: Description and Operation

Fig. 1 Exploded view of Rochester 1ME carburetor:

Fig. 2 Exploded view of Rochester 1MEF carburetor (Part 1 of 2):

Fig. 2 Exploded view of Rochester 1MEF carburetor (Part 2 of 2):

The Monojet 1ME, Fig. 1, is a single barrel, downdraft carburetor using a triple venturi and plain fuel tube nozzle to draw fuel from an integral float bowl. Fuel enters the carburetor through a pleated paper type fuel filter with an integral check valve. Fuel level within the float bowl is maintained by a plastic composition float acting on a viton tipped needle valve.
Fuel flow in the main metering circuit is controlled by a main well air bleed and variable orifice main jet. Power enrichment is provided by a mechanically operated metering rod connected to the throttle linkage, which varies the opening of the main jet depending upon throttle position.
1MEF carburetors, Fig. 2, include a metering rod adjusting screw located in the air horn that controls the position of the metering rod in the jet. This screw provides wide open throttle mixture adjustment and is preset at the factory. During normal service the position of this screw should not be altered, as increased exhaust emissions and engine damage may result.
Compensation enrichment during rapid throttle opening is provided by an acceleration pump plunger which draws fuel into the pump bore from the main float bowl. When the throttle is opened, the pump plunger pressurizes fuel within the pump bore, overcoming check ball spring tension and causing a stream of fuel to be delivered into the intake air stream. When pressure in the pump bore decreases, spring tension forces the pump check ball into its seat, preventing further injection of fuel. Then, as the throttle is closed, the pump plunger is raised and additional fuel is drawn into the pump bore through a slot milled into the side of the bore.
The idle/transition circuit consists of upper and lower idle air bleeds, idle and off idle discharge ports and an adjustable metering needle. At idle, vacuum created at the idle discharge port below the throttle plate draws air into the circuit through the upper idle air bleed. Vacuum created by air passing through the idle circuit draws fuel into the circuit from the main jet well through an idle tube. Fuel drawn into the circuit is further emulsified by the lower idle air bleed, and the emulsified mixture is regulated by the metering needle and discharged through the idle discharge port. As the throttle is opened, vacuum at the idle discharge port decreases and vacuum at the transition discharge port increases. Emulsified fuel is then drawn into the intake air stream through the transition port and regulated by the idle channel restriction.
An integral automatic choke mechanism is used which includes an electrically heated choke coil, linkage actuated fast idle cam and externally mounted choke pull-off. When the engine is cold, the choke blade is closed and fast idle cam set by choke coil tension after fully depressing, then releasing the accelerator pedal. If properly adjusted, this mechanism allows the engine to start and run at fast idle without further modulation of the accelerator pedal. When the engine starts, the vacuum operated pull-off opens the choke blade slightly to allow sufficient air into the carburetor to prevent flooding. As the engine runs, battery voltage applied to the choke coil warms the coil, which then expands and opens the choke blade. Once the choke blade begins to open, quickly depressing, then releasing the accelerator pedal will release the fast idle cam and allow the engine to run at normal curb idle speed.