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 .
When the engine in your Toyota 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 effect as keeping your foot slightly depressed on the gas pedal. The idle speed adjusting screw contacts a lever (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.
IDLE SPEED AND MIXTURE ADJUSTMENTS
See Figures 1, 2, 3, 4, 5, 6, 7, 8 and 9
Perform the following adjustments with the air cleaner in place. When adjusting the idle speed and mixture, the gear selector should be placed in Drive (D) on 1970-73 models equipped with an automatic transmission. Be sure to set the parking brake and block the front wheels. On all trucks equipped with manual transmissions and all 1974 automatics, adjust the idle speed with the gearshift in Neutral (N).
- Run the engine until it reaches normal operating temperature. Stop the engine.
- Connect a tachometer to the engine as detailed in the manufacturer's instructions.
- Remove the plug and install a vacuum gauge in the manifold vacuum port by using a suitable metric adapter.
- Start the engine and allow it to stabilize at idle.
- Turn the mixture screw in or out, until the engine runs smoothly at the lowest possible engine speed without stalling.
- Turn the idle speed screw until the vacuum gauge indicates the highest specified reading (see the "Vacuum At Idle" chart) at the specified idle speed. (See the "Tune-Up Specifications'' chart at the beginning of the section).
- Tighten the idle speed screw to the point just before the engine rpm and vacuum readings drop off.
- Remove the tachometer and the vacuum gauge. Install the plug back in the manifold vacuum port. Road-test the vehicle.
- In some states, emission inspection is required. In such cases, you should take your truck to a diagnostic center which has an HC/CO meter, and have the idle emission level checked to be sure that it is in accordance with state regulations. Starting 1974, CO levels at idle are given on the engine tune-up decal under the hood.
The idle speed and mixture should be adjusted under the following conditions: the air cleaner must be installed, the choke fully opened, the transmission should be in Neutral (N), all accessories should be turned off, all vacuum lines should be connected, and the ignition timing should be set to specification.
- Start the engine and allow it to reach normal operating temperature (180°F).
- Check the float setting; the fuel level should be just about even with the spot on the sight glass. If the fuel level is too high or low, adjust the float level. (See Fuel System ).
- Connect a tachometer in accordance with the manufacturer's instructions. However, connect the tachometer positive (+) lead to the coil Negative (-) terminal. Do NOT hook it up to the distributor or positive (+) side; damage to the transistorized ignition will result. In some cases there is a service connector at the igniter and the tachometer may be connected to this.
- Adjust the speed to the highest rpm it will attain with the idle mixture adjusting screw.
- Set the rpm to the idle mixture speed of 900 rpm, by turning the idle speed adjusting screw. You may have to repeat Steps 4 and 5 a few times until the highest idle reached in Step 4 will go no further.
- Now set the speed to the initial idle speed of 850 plus or minus 50 rpm, by turning the idle mixture adjusting screw in (clockwise).
- Disconnect the tachometer.
See Figures 10, 11, 12, 13 and 14
Use the same procedure as given for 1975-77 models, described above. However, substitute different idle mixture and idle speeds as specified below:
Certain models may have an idle limiter cap on the idle adjusting screw; if so, use pliers to break it off. Be sure to install a new cap after adjustment.
To meet U.S. emissions regulations, the idle mixture adjusting screw on the later models covered here is preadjusted and plugged by Toyota. When troubleshooting a rough idle, check all other possible causes before attempting to adjust the idle mixture; the plug should not be removed and the adjusting screw tampered with in the course of a normal tune-up. Toyota recommends all mixture adjustments be handled by a professional mechanic equipped with the proper emissions test equipment. If all other trouble causes have been checked, then on 1978-83 models the carburetor must be removed while the plug is removed form the mixture screw hole. Plug all vacuum ports to keep metal chips out before drilling. After the plug is removed, remove the mixture screw to inspect the tip for wear, and blow out the hole with compressed air. Reinstall the adjusting screw by screwing it in fully until it just seats, then unscrewing it 2 1 / 2 full turns (4 turns on 1983 models). Reinstall the carburetor and proceed with the mixture adjustment only if necessary. 1984-88 models require an HC/CO meter for mixture adjustments.