AutoZone 1987 Oldsmobile 98 3.8L FI OHV 6cyl | Repair Guides | Engine Performance And Tune-up | Idle Speed And Mixture Adjustment | Carbureted Vehicles | AutoZone.com

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    Toyota Cressida/Corona/Crown/MarkII 1970-1982 Repair Guide

    Carbureted Vehicles

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    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 .

    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 four-stroke 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 car. After you start the engine and put the transmission in gear, you depress the gas pedal to start the car moving. What you actually are doing when you depress the gas pedal is opening the throttle plates 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. The 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 will 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. The procedures given in this section will tell how to set the idle mixture adjusting screw.

    IDLE SPEED & MIXTURE



    1970-74 Models


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    Fig. Fig. 1 Carburetor adjustment locations-8R-C and 18R-C engines



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    Fig. Fig. 2

    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 cars equipped with manual transmissions and all 1974 automatics, adjust the idle speed with the gearshift in Neutral (N).

    1. Run the engine until it reaches normal operating temperature. Stop the engine.
    2.  
    3. Connect a tachometer to the engine as detailed in the manufacturer's instructions.
    4.  
      1. On models having a conventional ignition system, one lead (usually black) goes to a good chassis ground. The other lead (usually red) goes to the distributor primary side of the coil (the terminal with small wire running to the distributor body).
      2.  
      3. On models with transistorized ignition, connect one lead (usually black) of the tachometer to a good chassis ground. Connect the other lead (usually red) to the negative (-) coil terminal, NOT to the distributor or positive (+) side. Connecting the tach to the wrong side will damage the switching transistor.
      4.  

    5. Remove the plug and install a vacuum gauge in the manifold vacuum port by using a suitable metric adapter.
    6.  
    7. Start the engine and allow it to stabilize at idle.
    8.  
    9. Turn the mixture screw in or out, until the engine runs smoothly at the lowest possible engine speed without stalling.
    10.  
    11. 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).
    12.  
    13. Tighten the idle speed screw to the point just before the engine rpm and vacuum readings drop off.
    14.  
    15. Remove the tachometer and the vacuum gauge. Install the plug back in the manifold vacuum port. Road test the vehicle.
    16.  
    17. In some states, emission inspection is required. In such cases, you should take your car 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.
    18.  

    1975-77 Models


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    Fig. Fig. 3 Carburetor adjustment locations-20R and 22R engines



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    Fig. Fig. 4 Checking the fuel level through the sight glass on the carburetor

    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.

    1. Start the engine and allow it to reach normal operating temperature (180°F).
    2.  
    3. 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 ).
    4.  
    5. 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.
    6.  
    7. Adjust the speed to the highest rpm it will attain with the idle mixture adjusting screw.
    8.  
    9. Set the rpm with the idle speed adjusting screw to:
    10.  

    20R: 900 rpm

    4M (auto. trans.): 820 rpm

    4M (man. trans.): 870 rpm

    1. Repeat steps four and five until the highest rpm can be reached with the mixture screw and then readjust to rpm in Step 5.
    2.  
    3. Now set the speed by turning the idle mixture adjusting screw in (clockwise), to the initial idle speed of:
    4.  

    20R: 850 + 50 rpm

    4M (man. trans.): 800 rpm

    4M (auto. trans.): 750 rpm

    1. Disconnect the tachometer.
    2.  

    1978-82 Models


    Click image to see an enlarged view

    Fig. Fig. 5 Break off the idle limiter cap with a pair of pliers (if equipped)-20R and 22R engines



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    Fig. Fig. 6 Cap or plug all of the vacuum ports prior to drilling the plug-carburetor shown on its side



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    Fig. Fig. 7 Carefully use a drill on the plug; only 1mm of clearance is between the plug and top of the screw



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    Fig. Fig. 8 Carburetor adjustment locations-M series engines

    Use the same procedure described for 1975-77 models. 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 US 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 the carburetor must be removed and the plug removed from 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. Reinstall the carburetor and proceed with the mixture adjustment, if necessary.

    For idle speeds (Step 5), use the following specifications:

    1978-79 4M: 820

    1978 20R: 850

    1979 20R: 870 Manual, 920 Automatic

    1980 22R: 750 4 speed Manual

    920 Automatic 1980 22R California models. Idle speed adjusted with idle speed screw only

    800 Manual

    850 Manual 1981-82 22R

    700 Manual

    750 USA-Auto

    850 Canada-Auto For idle speed adjusted by mixture screw (Step 6), use the following figures:

    1978-79 4M: 750

    1978 20R: 800

    1979 20R: 800 Manual, 850 Automatic

    1980 22R: 700 4 speed Manual

    (exc. Calif.) 800 5 speed Manual

    850 Automatic

     
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