Nissan Z - ZX 1970-1988 Repair Guide

Idle Speed and Mixture Adjustments

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CARBURETED ENGINES



This section contains only tune-up adjustment procedures for carburetors. Descriptions, adjustments and overhaul procedures for carburetors can be found in the Fuel System section.

When the engine is running, the air/fuel mixture from the carburetor is being drawn into the engine by a partial vacuum which is created by the movement of the pistons downward on the intake stroke. The amount of air/fuel mixture that enters the engine is controlled by the throttle plates at the bottom of the carburetor. When the engine is not running, the throttle plates are closed, completely blocking off the bottom of the carburetor. The throttle plates are connected by the throttle linkage to the accelerator pedal in the passenger compartment. When the pedal is depressed, the throttle plates in the carburetor open to admit more air/fuel mixture to the engine.

When the engine is not running, the throttle plates are closed. When the engine is idling, it is necessary to have the throttle plates open slightly. To prevent having to hold your foot on the pedal when the engine is idling, an idle speed adjusting screw is added to the carburetor linkage.

The idle adjusting screw contacts a lever (throttle lever) on the outside of the carburetor. When the screw is turned, it either opens or closes the throttle plates of the carburetor, raising or lowering the idle speed of the engine. This screw is called the curb idle adjusting screw.

A special mixture circuit is incorporated into the carburetor to enable the engine to run smoothly at idle. The circuit is controlled by the mixture screw, which determines the amount of fuel admitted at idle.

1970-72 Models

See Figure 1

  1. Connect a tachometer, according to the manufacturer's instructions.
  2.  
  3. Run the engine until it reaches operating temperature. Remove the air cleaner.
  4.  
  5. Apply an air flow meter or other carburetor air measuring tool for 1-2 seconds to one of the carburetor air horns, holding it vertically. Then, check the flow of the other carburetor. Adjust the throttle adjusting screws until the airflow at idle is equal. Adjust the opener adjusting screws until the idle speed is 750 rpm.
  6.  
  7. Disconnect the control vacuum tube from the manifold connector and connect the servo diaphragm vacuum tube in it's place, in order to apply full vacuum to the diaphragm.
  8.  
  9. Adjust the opener adjusting screw to 1,200 rpm.
  10.  



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Fig. Fig. 1: Throttle linkage and carburetor component locations-1970-72 models

  1. Using the flow meter as described above, adjust the balance screw so that the airflow is equal for each carburetors.
  2.  
  3. Unfasten then reconnect the servo diaphragm vacuum tube where it is connected to the manifold. Check the rpm and flow rates; then, if necessary, readjust as in Steps 5 and 6.
  4.  
  5. Return the vacuum hoses to their original positions, disconnect the diaphragm tube from the manifold and reconnect it to the control valve, then reconnect the control valve hose to the manifold connection.
  6.  
  7. If a CO meter is available, adjust CO the as follows:
    1. Disconnect the air pump belt so that the pump will be inoperative.
    2.  
    3. Gently tighten the idle mixture adjusting nuts, located under the carburetors, until they hit their stops.
    4.  
    5. Turn both nuts equally outward until the CO is 5-7%.
    6.  
    7. Reconnect the air pump belt.
    8.  

  8.  

1973-74 Models

See Figures 2, 3 and 4

  1. Connect a tachometer, according to the manufacturer's instructions.
  2.  
  3. Run the engine until it reaches normal operating temperature. Remove the air cleaner.
  4.  
  5. Loosen the throttle adjusting screw. On 1974 models equipped with an automatic transmission, loosen the throttle opener adjusting screw as far as possible, without removing it.
  6.  
  7. Adjust the idle speed to 750 rpm, using the idle speed adjusting screw. On automatic transmission equipped models, block the drive wheels, securely apply the parking brake and place the transmission in DRIVE. Turn the idle speed adjusting screw until the tachometer reads 600 rpm. Return the selector to the NEUTRAL position.
  8.  

On automatic transmission equipped 260 Z models, proceed to Step 6, as these vehicles do not employ a throttle opener control valve.

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Fig. Fig. 2: Throttle linkage and carburetor components-1973-74 models



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Fig. Fig. 3: Use a flow meter to measure the air flow into the carburetor



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Fig. Fig. 4: Fast idle screw and EGR valve-1974 models

  1. Disconnect the vacuum control valve-to-servo diaphragm hose and the control valve-to-intake manifold hose.
  2.  
  3. Connect a longer hose of the same diameter between the servo diaphragm-to-manifold connector.
  4.  
  5. On all models, except the 1974, 260 Z equipped with an automatic transmission, turn the throttle opener adjusting screw until the tachometer reads 1,400 rpm. On 260 Zs equipped with an automatic transmission, do the same with the fast idle adjusting screw.
  6.  
  7. Apply a flow meter briefly to the front carburetor intake. Turn the adjusting screw on the flow meter and align the upper end of the float with the scale. Then, tighten the screw.
  8.  
  9. Apply the meter to the rear carburetor air intake and turn the balance adjusting screw, until the float in the meter is aligned with the scale.
  10.  
  11. Reinstall the air cleaner, connecting the vacuum motor to the temperature sensor with the vacuum hose.
  12.  
  13. Readjust the engine speed to 1,400 rpm as in Step 6.
  14.  
  15. Raise the engine speed briefly to 3,000 rpm. Then, raise it to 1,700 rpm with the throttle opener or fast idle screw (depending on the model as specified above). Finally, gradually lower the speed to 1,400 rpm.
  16.  
  17. Disconnect the air pump check valve hose and plug the check valve.
  18.  
  19. Using a CO meter, adjust the CO to 1.0-1.6% for models with a manual transmission or 0.6-1.2% for those with an automatics (in Neutral). This adjustment is made at the idle mixture adjusting screw.
  20.  
  21. On all vehicles, except the 260 Z (equipped with an AT), disconnect the servo diaphragm vacuum for 2-3 seconds, then reconnect it. Make sure that the rpm returns to 1,400; if not, readjust it at the throttle opener adjusting screw.
  22.  
  23. On all models, except the 1974 260 Z, remove the long vacuum hose and reconnect the hoses, so that one runs from the manifold-to-throttle opener control valve and the other from the control valve-to-servo. On the 1974 260 Z with an automatic transmission, turn the fast idle screw outward until there is a clearance of 2mm between the lever and the tip of the screw.
  24.  
  25. Race the engine several times to verify that the idle speed is correct. Readjust it if necessary.
  26.  
  27. Unplug the check valve and reconnect the hose.
  28.  
  29. Measure the CO percentage and make sure that it is below 2.7%.
  30.  

FUEL INJECTED ENGINES



The 1975 and later models have electronic fuel injection as standard equipment. The only regular tune-up maintenance required with this system is an idle speed check; 1975-79 manual transmission equipped models also may require a dashpot adjustment. The idle mixture is not adjustable on 1975-76 models; it can be adjusted on 1977 and later models, but requires the use of a CO meter.

It is unlikely you will ever have need to adjust the mixture and, because of the expense of a CO meter, it is suggested that any mixture adjustments be referred to your dealer or a qualified mechanic with access to the proper equipment.

Idle Speed

1975-83 MODELS

See Figures 5, 6 and 7

  1. Warm the engine to normal operating temperature, either by driving the car or by allowing it to idle.
  2.  
  3. When warm, continue to run the engine with the hood open for about five minutes at 2,000 rpm. If the car has air conditioning, it should be OFF .
  4.  
  5. Turn OFF the engine, then disconnect and plug the vacuum hose at the distributor vacuum controller.
  6.  

On 1982-83 models, disconnect the gray electrical harness from the distributor.

  1. Connect a tachometer, according to the manufacturer's instructions.
  2.  
  3. Start the engine and race it 2-3 times under no load, then allow it to run at idle.
  4.  
  5. Set the parking brake and block the front wheels. Shift the manual transmission into Neutral or the automatic transmission into DRIVE.
  6.  



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Fig. Fig. 5: Idle speed screw-1975-80 models



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Fig. Fig. 6: Idle speed screw-1981-82 models



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Fig. Fig. 7: Idle speed screw-1983 models

  1. Adjust the idle speed to specifications by turning the idle speed screw, which is located in the throttle chamber, next to the distributor.
  2.  

The idle speed for both transmissions should be 550-750 rpm.

  1. Using a timing light, check and/or adjust the ignition timing to 6-10° BTDC.
  2.  
  3. Operate the engine at 2,000 rpm for 2 minutes under no-load.
  4.  

Make sure the inspection lamp on the control unit goes ON and OFF more than 5 times in 10 seconds.

  1. Turn the engine OFF and reconnect the vacuum hose.
  2.  

On 1982-83 models, reconnect the grey harness to the distributor.

  1. Start the engine and race it 2-3 times, then allow it to idle.
  2.  
  3. Check and/or adjust the idle speed to 600-800 rpm.
  4.  

1984-88 MODELS

See Figures 8, 9 and 10

  1. Start the engine and run it until it reaches normal operating temperature. The water temperature indicator should point to the middle of the gauge. Make sure all of the accessories are turned OFF .
  2.  

Idle speed checking and adjustment is performed with the automatic transmission in DRIVE, so make sure the parking brake is fully engaged, and both the front and rear wheels are blocked with wheel chocks.

  1. Stop the engine, then connect a tachometer, according the the manufacturer's instructions.
  2.  
  3. If not equipped with a turbocharger, disconnect the idle-up solenoid valve harness connector.
  4.  
  5. Start the engine and race it 2-3 times, under no load, then return it to idle speed.
  6.  
  7. Check and adjust the idle speed as necessary by turning the idle speed adjusting screw (automatic transmission in DRIVE and the manual transmission in NEUTRAL).
  8.  

If not equipped with a turbocharger, set the idle speed to 650-750 rpm at sea level or 600-700 rpm at high altitude. If equipped with a turbocharger, set the idle speed to 650-750 rpm (manual transmission) or 600-700 (automatic transmission).

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Fig. Fig. 8: Idle speed screw-1984-88 non-turbo models



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Fig. Fig. 9: Idle-up solenoid-1984-88 non-turbo models



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Fig. Fig. 10: Idle adjust screw and AAC valve-1984-88 turbo models

  1. If equipped with a turbocharger, perform the following:
    1. Stop the engine and disconnect the Auxiliary Air Control (AAC) valve harness connector.
    2.  
    3. Start the engine.
    4.  
    5. Adjust the engine speed by turning the idle speed screw. The idle speed should be 650 rpm (manual transmission) or 600 rpm (automatic transmission).
    6.  
    7. Stop the engine.
    8.  
    9. Reconnect the Auxiliary Air Control (AAC) valve harness connector and start the engine.
    10.  

  2.  
  3. If not equipped with a turbocharger, stop the engine and reconnect the idle-up solenoid harness connector.
  4.  
  5. Using a timing light, start the engine and check the timing; it should be 18-22° BTDC on all except the 1987-88 turbocharged models. On 1987-88 models equipped with a turbocharged engine, the timing should be 13-17° BTDC. If not, adjust the distributor as required.
  6.  
  7. If a timing adjustment was performed, recheck the idle speed.
  8.  

Dashpot
1975-79 MODELS WITH MANUAL TRANSMISSION

See Figures 11 and 12

  1. Connect a tachometer according to the manufacturer's instructions.
  2.  
  3. Start the engine and run it at 2,000 rpm, under no load. If equipped with air conditioning, turn it OFF .
  4.  
  5. The clearance between the idle setscrew (preset at the factory) and the throttle lever should be 0.08 in. (2mm).
  6.  
  7. The dashpot plunger, when fully extended, should be in contact with the throttle lever.
  8.  
  9. If adjustment is necessary, loosen the dashpot mounting nut and turn the dashpot assembly until the clearance is correct. Tighten the nut and recheck the clearance.
  10.  



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Fig. Fig. 11: Dashpot measurement for 1970-78 models. If adjustment is needed, turn the locknut (indicated by the arrow)



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Fig. Fig. 12: Dashpot measurement for 1979 models. If adjustment is needed, turn the locknut (indicated by the arrow)

 
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