Carburetor: Description and Operation: 5740

Fig. 1 Motorcraft model 5740 2 barrel carburetor:

Fig. 2 Idle fuel shutoff solenoid system. 1985 shown, 1986 similar:

Fig. 3 Idle Speed Control (ISC) assembly:

This carburetor, Fig 1, is a two barrel unit having four basic metering systems: choke system, main metering system, acceleration system and idle system. On some applications, a power enrichment system is also used to provide improved performance during moderate to heavy acceleration.

An idle fuel shutoff solenoid system, Fig 2, is used to eliminate dieseling when the engine is shut off. When the ignition switch is in the On position, the idle fuel shutoff solenoid is energized, allowing fuel to flow through idle system of carburetor. When the ignition switch is turned to the Off position, the solenoid is de-energized and fuel flow through the carburetor idle system is blocked.

A dashpot, mounted on the carburetor, helps to control hydrocarbon emissions during deceleration by slowing the closing of the throttle plates. As the throttle plates close, the throttle lever applies pressure on the dashpot plunger. This causes air in the dashpot chamber to escape, allowing the throttle plates to slowly return to the idle position.

The altitude compensation system is used to improve driveability and reduce emissions at high altitude. The system consists of an altitude barometric switch, a bleed solenoid and associated wiring to the carburetor. The system operates only when the vehicle is operated at altitudes above approximately 4000 feet.

On certain applications, a Wide Open Throttle (WOT) A/C cutout switch is used to electrically disengage the A/C compressor clutch at WOT position.

The carburetor-mounted Idle Speed Control (ISC), Fig 3, is a throttle positioner which uses direct manifold vacuum to maintain a specified idle speed for various engine loading conditions. As manifold vacuum decreases, the ISC plunger moves outward to increase carburetor throttle plate opening and return engine to a specified speed.