This section contains only carburetor adjustments as they normally apply to engine tune-up. Descriptions of the carburetor and complete adjustment procedures can be found in Fuel System of this repair guide.
When the engine in your Nissan is running, air/fuel mixture from the carburetor is being drawn into the engine by a partial vacuum which is created by the downward movement of the pistons on the intake stroke of the 4-stroke cycle of the engine. The amount of air/fuel mixture that enters the engine is controlled by throttle plates in the bottom of the carburetor. When the engine is not running, the throttle plates are closed, completely blocking off the bottom of the carburetor from the inside of the engine. The throttle plates are connected, through the throttle linkage, to the gas pedal in the passenger compartment of the truck. After you start the engine and put the transmission in gear, you depress the gas pedal to start the truck moving. What you actually are doing when you depress the gas pedal is opening the throttle plate in the carburetor to admit more of the air/fuel mixture to the engine. The further you open the throttle plates in the carburetor, the higher the engine speed becomes.
As previously stated, when the engine is not running, the throttle plates in the carburetor are closed. When the engine is idling, it is necessary to open the throttle plates slightly. To prevent having to keep your foot on the gas pedal when the engine is idling, an idle speed adjusting screw was added to the carburetor. This screw has the same affect as keeping your foot slightly depressed on the gas pedal. The idle speed adjusting screw contacts the throttle lever on the outside of the carburetor. When the screw is turned in, it opens the throttle plate on the carburetor, raising the idle speed of the engine. This screw is called the curb idle adjusting screw, and the procedures in this section tell you how to adjust it.
Since it is difficult for the engine to draw the air/fuel mixture from the carburetor with the small amount of throttle plate opening that is present when the engine is idling, an idle mixture passage is provided in the carburetor. This passage delivers air/fuel mixture to the engine from a hole which is located in the bottom of the carburetor below the throttle plates. This idle mixture passage contains an adjusting screw which restricts the amount of air/fuel mixture that enters the engine at idle.
See Figures 1 and 2
- Start the engine and run it until it reaches operating temperature.
- Allow the engine idle speed to stabilize by running the engine at idle for at least 1 minute.
- If it hasn't already been done, check and adjust the ignition timing to the proper setting.
- Turn OFF the engine and connect a tachometer to the engine.
- Disconnect and plug the air hose between the three way connector and check valve, if equipped. Start the engine. With the transmission in N , check the idle speed on the tachometer. If the reading on the tachometer is correct, turn the idle adjusting screw clockwise to increase the idle speed and counterclockwise to decrease it.
- With an automatic transmission in D (wheels chocked and parking brake applied) or a manual transmission in N , turn the mixture screw out until the engine rpm starts to drop due to an overly rich mixture.
- Turn the screw in past the starting point until the engine rpm start to drop because of a too lean mixture. On 1975-77 models, turn the mixture screw in until the idle speed drops 60-70 rpm with manual transmission, or 15-25 rpm with automatic transmission (in D ). On 1978 models, the rpm drop should be 45-55 rpm for all trucks. For 1979-81 models the rpm drop should be 45-55 rpm with manual transmission, or 25-35 rpm with automatic transmission (in D ). If the mixture limiter cap will not allow this adjustment, remove it, make the adjustment, and reinstall it. Go on to Step 10 for 1975-81 trucks.
- On 1970-74 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 1970-74 models.
- Install the air hose. If the engine speed increases, reduce it with the idle speed screw.
To be sure that the vehicle is complying with emission laws, have the exhaust checked with a CO meter. The percentages of CO should be 3% for 1970-71, 2% 1972, 1.5% 1973-74, 2% 1975-77, and 1% 1978-81 at idle speed.Idle limiter caps are installed on the mixture adjusting screws so that an incorrect adjustment cannot be made. If a satisfactory idle cannot be obtained within the range of the limiter caps, remove them and make the adjustment as outlined above. Reinstall the limiter caps so that the cap can be turned only1/8of a turn counterclockwise before it reaches the stop. Have the engine checked with a CO meter after making the adjustment.
See Figures 3 and 4
These models require the use of a CO meter to adjust their mixture ratios, therefore only idle speed adjustments are given.
- Connect a tachometer according to the manufacturer's instructions.
- Turn all the accessories and lights OFF . Make sure that the wheels are straight ahead on models with power steering.
- Run the engine at 2000 rpm for 2 minutes with the transmission in P or N .
- Run the engine at normal idle speed for 1 minute in P or N .
- Check the idle speed with the automatic transmission in D , wheel blocked and the parking brake on, use the figures provided on your underhood sticker. If the indicated idle speed does not agree with the specified speed, adjust the idle by turning the throttle adjusting screw at the carburetor.