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 into the engine is controlled by the throttle plate(s) in the bottom of the carburetor. When the engine is not running the throttle plate(s) is closed, completely blocking off the bottom of the carburetor from the inside of the engine. The throttle plates are connected by the throttle linkage to the accelerator pedal in the passenger compartment of the vehicle. When you depress the pedal, you open the throttle plates in the carburetor 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 was 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.
The 1980 model Datsun/Nissan require a CO meter to adjust their mixture ratios, therefore, no procedures concerning this adjustment are given. Also, many California model Datsun/Nissan have a plug over their mixture control screw. It is suggested that in both of these cases, mixture adjustment be left to a qualified technician.
- Start the engine and allow it to run until it reaches normal operating temperature.
- Allow the engine idle speed to stabilize by running the engine at idle for at least two minutes.
- If you have not done so already, check and adjust the ignition timing to the proper setting.
- Shut off the engine and connect a tachometer as per the manufacturer's instructions.
- Disconnect and plug the air hose between the three-way connector and the check valve, if equipped. On 1980 models with the Z20S engine, disconnect the air induction hose and plug the pipe, also disconnect and plug the vacuum hose at the distributor. With the transmission in Neutral, check the idle speed on the tachometer. If the reading is correct, continue on to Step 6 for 1973-79 models. For 1980 and later, and certain California models, proceed to Step 10 below if the idle is correct. If the idle is not correct, for all models, turn the idle speed adjusting screw clockwise with a screwdriver to increase idle speed or counterclockwise to decrease it.
- With the automatic transmission in Drive (wheels blocked and parking brake on) or the manual transmission in Neutral, turn the mixture screw out until the engine rpm starts to drop due to an overly rich mixture.
- Turn the screw until just before the rpm starts to drop due to an overly lean mixture. Turn the mixture screw in until the idle speed drops 60-70 rpm with manual transmission, or 15-25 rpm with automatic transmission (in Drive) for 1975-76 610 and 710 models; 45-55 rpm for all 1977 710's, and 1978-79 510's and 200SX's. If the mixture limited cap will not allow this adjustment, remove it, make the adjustment, and install it. Go on to Step 10 for all 1975-79 models.
- On 1973-74 models, turn the mixture screw back out to the point midway between the two extreme positions where the engine began losing rpm to achieve the fastest and smoothest idle.
- Adjust the curb idle speed to the proper specification, on 1973-74 models, with the idle speed adjusting screw.
- Install the air hose (if so equipped). If the engine speed increases, reduce it with the idle speed screw.