Toyota Tercel 1984-1994 Repair Guide

Carbureted Engines

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This section contains only adjustments as they normally apply to engine tune-up. Descriptions of the carburetor and fuel systems and complete adjustment procedures can be found in Fuel System .

When the engine in your Toyota is running, the amount of air/fuel mixture that enters the engine is controlled by the 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 inside the car. What you actually are doing when you depress the gas pedal is opening the throttle plate in the carburetor to admit more air/fuel mixture to the engine. The further you open the throttle plates in the carburetor, the higher the engine speed becomes.

To keep the engine 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 is included on the carburetor. This screw has the same effect as keeping your foot on the gas pedal - it holds the throttle plate open just a bit. When the screw is turned in, it opens the throttle, 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.

When you first start the car after an overnight period, the cold motor requires a different air-fuel mixture to run properly. Because of the different mixture, the idle speed must be higher during cold engine operation. This High Idle (sometimes called cold idle or fast idle) speed is also adjustable and should be checked periodically. If the high idle is too low, the car will bog and stall until it warms up. If the high idle is set too high, the engine is wasting fuel and suffering increased and premature wear.

Before performing any carburetor adjustments, ALL of the following conditions must be met:



All accessories are switched off
 
Ignition timing is set correctly and all vacuum lines are connected
 
Transaxle in Neutral and engine warmed up to normal operating temperature
 
Choke opened fully
 
Fuel level in carburetor sight glass at the correct level. If the level is too high or low, adjust the float level as explained in Fuel System .
 

IDLE SPEED ADJUSTMENT



See Figures 1, 2 and 3

Once all the above conditions are met, the curb idle can be adjusted by turning the adjusting screw or knob at the rear of the carburetor. Turn it clockwise to raise the idle and counterclockwise to lower it. Keep a close eye on the tachometer while turning the screw; sometimes a small change in the adjustor causes a big change in the idle speed. Adjust the idle speed to the rpm shown in the Tune-Up Specifications chart.



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Fig. Fig. 1: Check the fuel level in the float bowl



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Fig. Fig. 2: Idle speed adjustment screw on 3A-C carbureted engines



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Fig. Fig. 3: Idle speed adjustment screw on 3E carbureted engines

HIGH IDLE ADJUSTMENT



See Figures 4, 5, 6, 7 and 8

  1. Stop the engine and remove the air cleaner and its housing.
  2.  
  3. Plug the hot idle compensator hose. It's the hose that runs from the lower front part of the carburetor body to a small valve on the air cleaner housing. If not plugged, it will create a vacuum leak causing either a rough idle or stalling.
  4.  



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Fig. Fig. 4: Plugging the M port on the TVSV valve, 3A-C models

  1. On 3A-C models, find the vacuum valve with five hoses connected to it between the block and the firewall. One hose will be in a separate position and four will be in line. This is the Thermostatic Vacuum Switching Valve (TVSV). Of the four hoses in line, one will be tabled M ; it's usually the second one from the upper or outer end of the valve. Give the hose on the M port 1 / 2 turn and remove it from the port. Plug the port (not the hose) with an airtight plug to prevent vacuum leaks.
  2.  



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Fig. Fig. 5: Plugging the vacuum hose from the EGR valve

  1. On 3E models, disconnect and plug the Exhaust Gas Recirculation (EGR) valve hose.
  2.  



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Fig. Fig. 6: Disconnecting the EBCV connector

  1. Disconnect the Electronic Air Bleed Control Valve (EBCV) connector.
  2.  
  3. Hold the throttle plate open slightly (you can move the linkage with your fingers or pull gently on the accelerator cable) and move the choke plate to its fully closed position. Hold the choke closed as you release the throttle cable.
  4.  
  5. Start the engine but DO NOT move the gas pedal or the accelerator cable. The engine has been fooled into thinking it's cold - the choke is set and the high idle is engaged.
  6.  



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Fig. Fig. 7: Fast idle adjustment screw on 3A-C engines



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Fig. Fig. 8: Fast idle adjustment on 3E engines

  1. Adjust the high idle by turning the fast idle adjusting screw. Set the fast idle between 2,800 and 3,000 rpm.
  2.  


CAUTION
The engine is running at high speed. Beware of moving parts and hot surfaces.

  1. When the fast idle is set correctly, shut the engine off. Reconnect all hoses and electrical connections. Remove the plug from the hot idle compensator hose.
  2.  
  3. Reinstall the air cleaner housing and the filter; connect the hose.
  4.  

IDLE MIXTURE ADJUSTMENT



See Figures 9, 10, 11, 12, 13, 14, 15 and 16

To conform with Federal regulations, the idle mixture adjusting screw is adjusted at the factory and plugged with a steel plug by the manufacturer. Under normal conditions there should be no need to remove this plug.

When troubleshooting rough idle, check all other possible causes before attempting to adjust the idle mixture. Only if no other factors are found to be at fault should the idle mixture be adjusted. Since this repair involves removal of the carburetor, it is recommended that the car be thoroughly checked on an exhaust emissions analyzer as part of the diagnostic procedure before committing to the repair. If you perform this repair incorrectly or if the idle mixture is not the cause of your problem, you may cause the car to become uncertifiable under Federal and State or Provincial emission laws.

  1. Following the procedures given in Fuel System , remove the carburetor.
  2.  
  3. Using the following procedure remove the Mixture Adjusting Screw plug (MAS plug):
  4.  



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Fig. Fig. 9: Plug the vacuum ports to prevent the entry of steel particles when drilling

    1. Plug all of the carburetor ports to prevent the entry of steel particles when drilling.
    2.  
    3. Mark the center of the plug with a punch.
    4.  




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Fig. Fig. 10: Drilling the mixture adjustment screw (MAS) plug

    1. Drill a 6mm hole in the center of the plug. As there is only 1mm clearance between the plug and the screw below it, drill carefully and slowly to avoid drilling onto the screw. The drill may force the plug off at any time.
    2.  




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Fig. Fig. 11: Fully seat the mixture adjusting screw

    1. Through the hole in the plug, fully screw in the mixture adjusting screw with a screwdriver.
    2.  


Be careful not to damage the screw tip by tightening the screw too tightly.



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Fig. Fig. 12: Forcing the plug off with the drill

    1. Use a 7mm drill to force the plug off.
    2.  

  1. Remove the mixture adjusting screw.
  2.  



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Fig. Fig. 13: Inspect the mixture adjusting screw, 3A-C models shown



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Fig. Fig. 14: Mixture adjusting screw on 3E models

  1. Inspect the mixture adjusting screw as follows:
    1. Blow off any steel particles with compressed air.
    2.  
    3. If the drill has gnawed into the screw top or if the tapered portion is damaged, replace the screw.
    4.  

  2.  



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Fig. Fig. 15: On 3A-C models, fully seat the adjusting screw, then back out the specified number of turns

  1. Reinstall the mixture adjusting screw. On 3A-C models, fully seat the idle mixture adjusting screw and then back it out 3 1 / 4 turns (except Canada, 2 1 / 2 turns). On 3E models, screw in until the head is 0.138 in. (3.5mm) below the lower surface of the carburetor.
  2.  



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Fig. Fig. 16: Mixture adjusting screw depth on 3E models

Be careful not to damage the screw tip by tightening the screw too tight.

  1. Reinstall the carburetor on the engine. Reconnect the vacuum hoses to their proper locations.
  2.  
  3. Reinstall the air cleaner.
  4.  
  5. Adjust the idle speed and mixture as follows:
    1. Check the following initial conditions:

      The air cleaner is installed
       
      The engine is at normal operating temperature
       
      The choke is fully open
       
      All accessories are switch off
       
      All vacuum lines are connected
       
      The ignition timing is correct
       
      The transaxle is in neutral (N)
       
      The float level is correct
       
      The front wheels are pointed straight ahead (power steering equipped vehicles)
       

    2.  

    1. Start the engine
    2.  
    3. Turn the mixture adjustment screw slowly until the maximum idle speed is obtained. The preliminary adjustment of should be fairly close.
    4.  
    5. Set the idle speed by turning the idle speed adjusting screw. The idle mixture speed should be as shown on either the underhood Emissions Label or the Tune-Up Specifications chart.
    6.  
    7. Before moving to the next step, repeat adjustments until the maximum speed will not raise any further no matter how much the idle mixture adjusting screw is adjusted.
    8.  
    9. Final adjust to 650 rpm on 3A-C models and to 700 (M/T) or 900 (A/T) rpm on 3E models by turning the mixture adjusting screw.
    10.  
    11. Final adjust the idle speed to specification by turning the idle adjusting screw.
    12.  

  6.  

  1. Reinstall the mixture adjusting screw plugs. Remove the air cleaner and the EGR vacuum modulator bracket. With the tapered end of the plug facing inward, tap in the plug until it is even with the carburetor surface.
  2.  
  3. Reinstall the EGR vacuum modulator bracket and the air cleaner.
  4.  

 
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