MOTORCRAFT Model 740 2-Barrel: Description

The Motorcraft Model 740 is a 2-stage, dual venturi downdraft type carburetor. It features 5 basic metering systems: choke system, idle system, main metering system, acceleration system and power enrichment system. The carburetor also includes an altitude compensation system that operates between 2500-3000 feet, depending on engine calibration.

The choke system consists of a bi-metallic thermostatic coil with electric heater to improve cold engine starts. Fuel in the fuel bowl is maintained at a preset level by the float and needle. The idle system receives fuel from the fuel bowl to provide air/fuel mixture for idle and off-idle operation. As throttle opening and vacuum in carburetor venturi increase, the idle system tapers off and the main metering system begins operation.

The main metering system is divided into a primary main metering circuit and secondary main metering circuit. The primary circuit maintains the air/fuel ratio during normal operation. As the throttle plate reaches about 45°, the secondary throttle plate begins to open and the secondary metering circuit provides the fuel necessary to increase engine speed. When the throttle plates are quickly opened, the accelerator pump system supplies the extra fuel required until the main metering system can provide the required air/fuel mixture.

The power enrichment system is divided into primary and secondary circuits. The primary circuit is controlled by intake manifold vacuum. When intake manifold vacuum is high, the power valve is seated by spring pressure. When intake manifold vacuum decreases, the spring pressure overcomes the vacuum and the power valve is unseated to provide extra fuel to richen the air/fuel mixture. The secondary circuit operates by air velocity. As air velocity through secondary bore increases, low pressure draws the extra fuel into the air/fuel mixture to enrichen the mixture. The power enrichment system provides a richer mixture during heavy load operation.

All carburetors are equipped, with an idle fuel shut off solenoid. When the ignition switch is on, the solenoid is energized and fuel flows through idle system. When the ignition switch is turned off, the solenoid is de-energized and fuel flow through idle system is shutoff to prevent dieseling. Some models incorporate a decel idle fuel shutoff circuit within the solenoid to shutoff fuel flow to idle system when directed by the computer. This circuit is automatically canceled by the computer when necessary. Some models are also equipped with a wide open throttle A/C cutout switch to disengage the air conditioning compressor clutch at wide open throttle operation. This switch is mounted with an actuating arm that contacts the actuating pin on the fast idle lever at the wide open throttle position to de-energize the compressor clutch. The switch energizes the clutch when wide open throttle is canceled.