Carburetor - Hitachi Dfb 2-bbl Chevrolet Sprint I

COMPONENT TESTING

Note. Specific component testing information not available. Refer to any tests performed during ADJUSTMENTS or OVERHAUL procedures.

DESCRIPTION

Carburetor is a 2-barrel downdraft design with primary and secondary venturies. The choke system is fully automatic, using a thermowax element to regulate the choke plate position, depending on the engine coolant temperature.

The primary system is comprised of a choke, mixture control solenoid, fuel-cut solenoid and accelerator pump. The secondary system is comprised of a vacuum-actuated secondary vacuum break unit. The vacuum break is operated by vacuum from primary side of carburetor.

A switch vent solenoid valve is mounted on top of the float chamber. This vent is used to reduce evaporative emissions.

ADJUSTMENTS

Note. For all on-vehicle adjustments not covered in this article, see the TUNE-UP article in TUNE-UP section.

ACCELERATOR CABLE

Loosen accelerator cable lock nut. Adjust cable as necessary to bring cable play within specification. With engine cold and the choke on, cable play should be.40-.59" (10.2-15.0 mm). With engine at normal operating temperature and choke off, cable play should be.12-.19" (3.0-4.8 mm). (Scheme 1)

Adjusting Accelerator Cable. Scheme 1

FLOAT LEVEL

Note. Fuel level in float chamber should be within round mark at center of sight glass. If adjustment is required, use following procedure.

Remove and invert air horn. Measure distance between float and air horn without gasket. Float level should be.21-.25" (5.3-6.3 mm). Adjust float level by bending tab on float. (Scheme 2)

Adjusting Float Level. Scheme 2

Note. Perform the following adjustments on cold engine.

Scheme 3

Scheme 4

Scheme 5

1. Check choke plate for free movement by depressing with finger. Ensure choke valve is almost completely closed when ambient temperature is below 77°F (25°C) and engine is cold. Ensure choke valve-to-carburetor bore clearance is set to specifications shown in «CHOKE PLATE CLEARANCE SPECIFICATIONS»(/chevrolet/sprint/i-1984-1988/remont/testing-diagnostics/#carburetor-hitachi-dfb-2-bbl). (Scheme 3) CHOKE PLATE CLEARANCE SPECIFICATIONS Temperature: °F (°C) Clearance: In. (mm) Sprint 77 (25).004-.019 (.10-.48) 95 (35).03-.06 (.8-1.5) (Scheme 3): Checking Choke Plate Clearance Specification
2. If clearance is not within specifications, remove air cleaner assembly. Check strangler spring, choke piston and choke linkage operation. Lubricate linkage and choke plate as necessary. DO NOT remove riveted choke lever guide.
3. If clearance is still out of specification, remove carburetor and disconnect idle-up actuator from carburetor. Turn fast idle cam counterclockwise and insert a pin into holes on cam and bracket to lock cam in place. With cam locked in this position, bend the choke lever up or down as necessary. (Scheme 4) (Scheme 4): Adjusting Choke Lever
4. Disconnect choke piston hose at throttle chamber. Close choke plate using light finger pressure. Using a vacuum pump, apply vacuum to choke piston. Ensure that choke valve-to-carburetor bore clearance is.09-.10" (2.3-2.5 mm). (Scheme 5): Checking Choke Piston & Piston Rod Operation
5. With vacuum applied to choke piston, move choke piston rod into diaphragm with a small screwdriver. (Scheme 5) Ensure choke valve-to-carburetor bore clearance is.16-.18" (4.1-4.6 mm).

FAST IDLE CAM CHECK

Note. Ambient temperature must be 72-82°F (22-28°C) before performing check.

With engine cold, drain cooling system. Remove carburetor and place it where ambient temperature is 72-82°F (22-28°C) for one hour. After one hour, ensure mark on cam and center of cam follower are in alignment. (Scheme 6)

Checking Fast Idle Cam. Scheme 6

CHOKE UNLOADER

With choke valve closed, fully open throttle and check choke valve-to-carburetor bore clearance. Clearance should be.10-.12" (2.5-3.0 mm). To adjust unloader clearance, bend tab on unloader arm. (Scheme 7)

Adjusting Choke Unloader. Scheme 7

DISASSEMBLY

1. Disconnect 3 hoses and accelerator pump lever from carburetor air horn. Remove 5 air horn mounting screws and air horn from carburetor body. Remove float, needle valve, seat and filter from air horn.
2. Remove switch vent solenoid, choke thermowax element, choke pull-off, and accelerator pump from air horn. Remove idle-up actuator, fuel-cut solenoid valve and secondary diaphragm from carburetor main body.
3. Remove 2 springs and microswitch assembly from carburetor body. DO NOT remove microswitches from bracket. Remove lock plate and 2 float bowl drain plugs. Remove primary and secondary main jets from carburetor.
4. Remove air and fuel jets from air horn and carburetor body. Keep all jets in order for reassembly in original positions. Remove 4 throttle assembly retaining screws and remove throttle assembly from body.

CLEANING & INSPECTION

Note. DO NOT wash electrical components, choke thermowax element, diaphragms, gaskets or rubber components in carburetor cleaner.

1. Wash all parts in clean solvent and blow dry with compressed air. Blow through all passages with compressed air. DO NOT insert drills or wire into passages or jets to clean.
2. Check choke and throttle valves for smooth operation. Inspect air horn, main body and throttle body for cracks or breakage. Check accelerator pump rubber cup and mixture control solenoid valve seal for deterioration and damage. Check needle valve and seat for wear. Check all remaining seals and gaskets for wear and damage.

Scheme 8

Scheme 9

Scheme 10

1. Install new gasket to main body. Attach both ends of throttle lever return spring. (Scheme 8) Install throttle body to carburetor main body. Hook throttle valve end of return spring over boss on bottom of main body. (Scheme 8): Installing Throttle Lever Return Spring
2. Install throttle body retaining screws with correct style lock washer. (Scheme 9) Tighten mounting screws to 36-60 INCH lbs. (4-7 N.m). Install primary and secondary main jets. Install float bowl drain plug gaskets and drain plugs. Install drain plug lock plate. (Scheme 9): Positioning Throttle Body Lock Washers
3. Install float level gauge seal and level gauge. Install gauge with mark to the inside. Install microswitch assemblies and springs. Check microswitch adjustment. Install jets, air bleeds and plug in original position in carburetor main body. (Scheme 10) (Scheme 10): Installing Jets, Air Bleeds & Plug
4. Install check balls, weights and injector spring. Install accelerator pump piston return spring with "U" shaped end down. Install needle valve filter, seat, gasket, needle valve and float to air horn. Adjust float level as necessary. Install primary and secondary slow air bleeds in air horn.
5. Apply silicone grease to mixture control solenoid valve seal. Install new air horn gasket. Install air horn to main body. Install accelerator pump lever. Check accelerator pump lever for smooth operation.

Exploded View of Hitachi DFB 306 Carburetor Assembly. Scheme 11

SPECIFICATIONS

CARBURETOR ADJUSTMENT SPECIFICATIONS

See also:
• ADJUSTMENTS
• OVERHAUL