Carburetor - Hitachi Dhc 340 2-bbl Chevrolet Cab & Chassis C20

COMPONENT TESTING

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

APPLICATION

CHEVROLET & GMC CARBURETOR NO.

DESCRIPTION

Carburetor is a 2-barrel downdraft type with piston type accelerator pump. Carburetor consists of low speed (primary) barrel and high speed (secondary) barrel integrated into a single unit with a common fuel bowl.

Secondary throttle is actuated by a vacuum diaphragm when primary throttle is opened a pre-determined amount. Additional equipment includes a slow cut solenoid and power valve.

ADJUSTMENT

Note. For all on-vehicle adjustments, see appropriate TUNE-UP article.

FLOAT LEVEL

Fuel bowl is equipped with a fuel level sight glass. Line on sight glass indicates proper fuel level. Float level is not directly adjusted, but is controlled by needle valve clearance. If fuel level is incorrect, perform Float Drop adjustment.

FLOAT DROP

1. With fuel level sight glass removed and carburetor main body inverted, fully raise float. Measure clearance between float tang and needle valve stem. Needle valve should be bottomed in bore. (Scheme 2)
2. Clearance should be.059" (1.5 mm). If clearance is incorrect, adjust by bending float tang.

Float Drop (Level) Adjustment Raise float to measure clearance. Scheme 2

PRIMARY THROTTLE OPENING

1. Check primary throttle valve angle with choke completely closed. Adjust throttle angle by turning fast idle screw, making sure screw is all the way in before measuring clearance.
2. With choke closed, check clearance between throttle valve and throttle bore at center of throttle plate. Primary throttle valve clearance should be.050-.059" (1.28-1.51 mm) with throttle valve angle of 16°. (Scheme 3)

Primary Throttle Valve Adjustment Close choke fully when adjusting. Scheme 3

SECONDARY THROTTLE INITIAL OPENING

1. When primary throttle valve opens 47°, primary throttle lever tang contacts secondary throttle lock-out. Any further opening of throttle valve will force secondary throttle lock-out lever to actuate secondary throttle lever. Secondary throttle valve will begin to open. Check and adjust as follows.
2. Open primary throttle valve until primary throttle lever tang just contacts secondary throttle lock-out. Hold throttle in this position. Clearance between center of primary throttle valve and throttle bore should be.24-.30" (6.1-7.6 mm). If not, adjust by bending primary throttle tang. (Scheme 4)

Secondary Throttle Initial Opening Adjustment Measure clearance between center of primary throttle valve and throttle bore. Scheme 4

SECONDARY LOCK-OUT

1. Close primary throttle valve by turning throttle adjusting screw out.
2. With throttle valve completely closed, loosen lock nut on lock-out lever screw. Turn screw until it contacts return plate. Tighten lock nut. (Scheme 5)

Secondary Lock-Out Adjustment Perform adjustment with primary throttle valve completely closed. Scheme 5

DISASSEMBLY

1. With carburetor removed, remove main and assist throttle return springs. Disconnect accelerator pump lever.
2. Remove harness connector hanger from carburetor and disconnect automatic choke lead wire from connector. Remove fuel inlet pipe and filter.
3. Disconnect vent valve switch wire from connector. Remove circlip connecting choke rod to choke lever. Disconnect choke connecting rod.
4. Disconnect automatic choke vacuum hose. Remove 4 choke-to-air horn screws. Remove choke assembly. Remove circlip connecting vacuum break diaphragm to secondary throttle lever. Remove vacuum break diaphragm.
5. Separate main body from throttle body. Remove accelerator pump plunger assembly. Remove needle valve assembly.
6. Remove 3 float level cover screws. Remove cover, float level gauge, gasket and float assembly. DO NOT damage rubber seal or lose float collar.
7. Remove diaphragm cover retaining screws. Separate diaphragm cover, spring and diaphragm. DO NOT lose ball and spring.
8. Remove all jets from upper part of main body. Remove power valve. (Scheme 6) Remove pump set screw, weight and check ball.
9. Remove main jet plugs and primary and secondary main jets. Remove primary slow air bleed from air horn.
10. Further disassembly is not required. Primary and secondary throttle valves, and choke valve screws are staked in position. No attempt should be made to remove screws.

Location of Jets in Main Body Make sure jet wrench or screwdriver fits securely in slots of jets. Scheme 6

Air Horn

Inspect air horn for cracks and damage. Pay particular attention to mating surfaces. Check shaft holes for wear. Check choke valve and vacuum piston for smooth operation.

Main Body

1. Inspect and remove carbon deposits from inside main body. Inspect for cracks and damage, particularly on mating surfaces. Inspect threaded portion and head slots of jets for damage.
2. Check power valve for leaks. Check power valve rod for smooth operation and ensure that rod is not bent.
3. Inspect needle valve for sticking, dirt, and corrosion. Carefully check float for pin holes (leaks) and wear. Inspect filter in fuel inlet pipe for dirt, corrosion and damage.
4. Inspect accelerator pump plunger for damage and distortion. Check for smooth plunger movement within cylinder bore. Check accelerator pump boot for tears or damage.

Throttle Body

1. Check all ports for clogging. Inspect throttle valves for carbon deposits and wear. Check throttle shaft holes for wear.
2. Check mixture adjusting screw seating face for step wear. Check vacuum break diaphragm for deterioration and damage.

REASSEMBLY

To reassemble, reverse disassembly procedures and note the following

1. Be careful not to bend rod when installing power jet valve.
2. After accelerator pump is assembled, fill cavity with fuel. Depress accelerator pump and ensure fuel is injected smoothly. DO NOT bend piston connecting rod during assembly.

Exploded View of Hitachi DCH 340 2-Bbl. Carburetor. Scheme 7

SPECIFICATIONS

CARBURETOR ADJUSTMENT SPECIFICATIONS