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[Datsun 1200 encyclopedia]

Hitachi Twin Carb Adjustment

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Categories: Fuel System | Hitachi Twin Carb

The Hitachi variable venturi carburetors are of a simple elegant design. They are no diaphragms or rubber parts to wear out (at least not in the pre-1975 models). The throttle shafts can wear, so make sure they are fine, and other than that they just need simple cleaning and adjustment from time to time.


Oil Check

The oil should be checked every 3000 miles or 3 months, or whenever carburetor operation seems incorrect. Nissan recommends:

  • 20W engine oil
  • 10W-30 engine oil

Remove the oil cap Nut and check the oil level. Do not add oil if it is between the two lines:

20162.jpgAlbum 20162

The damper only holds 3cc of oil, so be careful when adding. Just a touch is all that is called for.

Baseline Settings

If you have rebuilt the carburetors, or have never run them, or they don't run very well at all, you should re-set them the baseline settings.

If installed, warm engine fully, then stop it, and remove the air cleaner.

Set screws to BASELINE

  1. Loosen both Throttle Adjusting screws (yellow arrows) and the Balance Adjusting screw (green arrow) -- to where they are no longer contacting the levers
    20176.jpgAlbum 20176
  2. Turn both Throttle Adjusting screws in until they contact the levers, then an additional 2 full turns after contact. This is your baseline setting.
  3. Make sure your dash choke knob is all the way in. Then push up on the nozzle assembly to ensure the settting. The arrow is pointing to the nozzle area
    20177.jpgAlbum 20177
  4. Screw the Idle Adjusting Nuts (yellow arrow) in tight
  5. Turn Idle Adjusting Nuts out per Hitachi chart

For example, with outside temperature at:

  • 86 degrees sea level: 2 turns
  • 50 degrees sea level: 3 turns
  • 50 degrees 5000 ft: 2-1/4 turns

Or this one:

Idle Speed & Mixture

For Flat-Top adjustment, see IMS

On the Hitachi variable venturi carburetor, the needle valve ensures a correct range of mixture, as long as the needle is seated correctly, linkages are correctly adjusted, and the piston is moving freely.

A. Warm engine fully, then stop it, and remove the air cleaner

B. Set screws to BASELINE as described above

C. Now we will set the idle "for real", not just at BASELINE which was just to get it running somewhat OK.

CAUTION: Make sure your dash choke knob is all 
the way in. Then push up on the nozzle assembly 
to ensure the settting. The arrow is pointing 
to the nozzle area.

20177.jpgAlbum 20177

D. Adjust Throttle Adjusting screws equally for 600 to 700 rpm. In other words, if it is idling too fast, turn both screws out 1/4 turn each. Repeat until the idle speed is 600 to 700 rpm

E. After the initial idle speed is set, SYNCHRONIZE both carbs

  1. Using carb synchronizer, for example Uni-syn, or STE Syncronizer, measure front carb airflow
  2. Put synchronizer on rear carb. Adjust rear carb Throttle Adjusting screw to the same airflow reading you saw on the front carb.

F. Turn Idle Adjusting Nuts equally for "best idle" (fastest smooth idle). Since you started with them tightened clockwise (lean) loosen them (anti-clockwise) to increase the engine speed. When engine starts to slow down, tighten them slightly until just before it slows down. You want the fastest rich idle. IMPORTANT: Adjust both to the same number of turns. Adjust them out 1/4 turn at a time. When it slows down, tighten them 1/8 turn. Keep repeating until you are certain it is the best idle.

G. Adjust Throttle Adjusting screws for 650 RPM. Repeat Synchronization. Repeat idle speed 650 RPM.

H. Turn Balance Adjusting screw just until contact is made (don't let it change the idle speed)

J. Bring revs to 2000 RPM. Let it idle. Double-check the idle speed.

K. Verify tuning with engine-running Piston Lifter check

WARNING: always do this to avoid a false mixture setting
and so protect the engine from leanout

Engine-running Piston Lifter check:

  1. Engine running,race the engine for 10 seconds, then let it drop backc to idle
  2. push up on the the lifter pin:
    18566.jpgAlbum 18566

    18567.jpgAlbum 18567
  3. When you raise the pin, the speed should go up slightly, then should drop. Release the pin, it should fall back to idle speed.
  4. Repeat for the 2nd carburetor

If both lifters (both carbs) affect the engine the same, syncronization is good.

If each lifters affects engine differently:

  • check float level while engine is running
  • if dynamic float level is OK, repeat synchronization

Static Piston Lift Check

Check the piston left every 3000 mile or 3 months, or whenever carburetor operation is unsatisfactory. Grit may cause the piston to stick, affecting performance.

  1. Remove air cleaner
  2. Remove oil cap nut
  3. Stick your finger in the bore. Slowly lift the piston to avoid splashing the oil.
  4. Lift piston until it stops
    20173.jpgAlbum 20173
  5. Drop piston. It should click against the venturi stop.
  6. Replace oil cap nut.
  7. Lift piston again this time against resistance of the oil. Piston should drop freely.
  8. If movement is sticky other otherwise not smooth, remove Suction chamber and carefully clean chamber and piston.

Float Level Verification Check

To check the float level, attach a sight glass to the drain plug. You can also use a length of clear plastic tubing.

  1. Remove drain plug. Catch gasoline in a cup.
  2. Take a 5/16" hose L-fitting. Fit clear 30mm plastic tubing pointing up, and jam the fitting into the drain hole. Make sure it is secure. Position it parallel to the bowl (straight up/down, taped to the side of the fuel bowl with the end of the tube higher than the top of the bowl)
    Nissan had a tool for this, number ST19200000 "Float level gauge"
  3. Start the engine and let it idle.
  4. Measure the level of fuel in the tube
    • 22-24 mm from fuel bowl (top) gasket
  5. Adjust float if too high or too low, then repeat this test.
  6. If level is correct, raise engine speed to 4000 RPM and note the fuel level.
    • If it is now incorrect, replace the fuel inlet "needle & seat"

Float Level Static Adjustment

When replacing a damage float or needle and seat, set the static level of the float. Or do this anytime the fuel level is incorrect.

(green arrow points to adjusting tab)
20170.jpgAlbum 20170

  1. Remove Float Chamber cover
  2. Place it upside down and level
  3. Level from top of brass lever to flange (without gasket) should be:
    11-12 mm
  4. If needed, adjust lever by carefully (and slightly) bending inner tab of brass lever.

23425.jpgAlbum 23425

Choke Cable Adjustment

Adjust the fast idle or "starter interlock", so that when the choke is engaged, it forces the throttle plate open slightly.

20171.jpgAlbum 20171

  1. Squeeze the choke lever as shown (nozzle will move down). The yellow connecting link will force the throttle open slightly.
  2. Measure the throttle plate opening
    0.6 mm
  3. Adjust the amount of opening by bending the link.

Throttle opens by 0.6 mm (measured between edge of plate and throttle bore)
20172.jpgAlbum 20172

Cleaning Suction Chamber

Remove the four bolts of the Dome Top (suction chamber).

The piston holds the oil, so keep upright.

CAUTION: Do not knock piston or jet. This is a precision assembly and a little jar can damage it.

20163.jpgAlbum 20163

CAUTION: Do not sand. It is a precision fit.

  • Clean with solvent (ether or carb cleaner)
  • Air dry to prevent dust or lint particles from sticking to surface
  • DO NOT oil inside of chamber nor edge of piston. It must be clean and dry for proper operation
  • Lightly oil the piston tube (a couple of drop)
  • Reinstall. If you turned the piston upside down, don't forget the nylon packing (washer).
  • Replace oil if you drained it from the piston tube

20174.jpgAlbum 20174

NOTE: There is no gasket between the Suction Chamber and the Throttle chamber.

Changing Needle Jet

My 1200 GX carbs uses needle M-75.

  • 16554-H2310 [M-75 ?]
  • 16554-H2316 from November 1971 M-83 [per Nismo Schematic]

Normally there is no reason to remove the needle jet. It can be thoroughly cleaned without removing.

20165.jpgAlbum 20165

If you wish to change the needle or replace a damaged needle (for example, having dropped the piston and bent the needle):

  1. remove screws holding Suction Housing and lift housing off
  2. lift piston up of the throttle body
  3. drain piston tube of oil
  4. remove set screw from side of piston
  5. carefully grab base of needle a ease out of piston.
    • If it won't come out, use needle nose pliers at base of needle. Do not scar, scratch or bend the needle.
    • when fitting new needle, be sure base of needle is flush with bottom of piston (not with the boss).

CRITICAL: All range of performance and economy rely of needle base being flush with piston face. It will go in further, but must not be allowed to. Use a straightedge to confirm.

The set screw looks like a traditional Hitachi downdraft jet screw, but has no hole in it. Just tighten slightly to fix the needle.

20175.jpgAlbum 20175

Nozzle Cleaning

The nozzle is a precision assembly located on the bottom of the carburetor.

20172.jpgAlbum 20172

Do not disassemble unless required. General cleaning can be done as follows:

  1. Remove fuel line at either end (at the nozzle or at the bottom of the float chamber).
  2. Let drain
  3. lift piston with finger
  4. spray carb cleaner (or ether) up the fuel hose into the nozzle. Spray should come out the venturi

20173.jpgAlbum 20173

Rebuild Kits

See Datto1000wagon's write-up: Tech Article on



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