Honda Accord/Civic/Prelude 1973-1983 Repair Guide

Idle Speed and Mixture Adjustment

Print

This section contains only adjustments which apply to engine tune-up - namely, idle speed and mixture adjustments. Descriptions of the carburetors used and complete adjustment procedures can be found in Fuel System .

Idle speed and mixture adjustment is the last step in any tune-up. Prior to making the final carburetor adjustments, make sure that the spark plugs, points and condenser (if used), dwell angle, ignition timing and valve clearance have all been checked, serviced and adjusted (if necessary). If any of these tune-up items have been overlooked, it may be difficult to obtain a proper adjustment.

All adjustments must be made with the engine fully warmed to operating temperatures.

CARBURETED MODELS



Civic 1170cc - Hitachi 2-bbl (1973)
  1. Set the parking brake and block the front wheels.
  2.  
  3. Turn the headlights ON and the cooling fan OFF .
  4.  
  5. From the radiator base, on the engine side, disconnect the electrical leads from either the cooling fan motor or the thermoswitch (screwed into the base of the radiator).
  6.  
  7. If equipped with a manual transaxle, position the shift selector in Neutral ; if equipped with a Hondamatic transaxle, position the shift selector to gear 1 .
  8.  
  9. From the carburetor, remove the limiter cap and turn the idle mixture screw counterclockwise, until engine speed drops, then, turn the idle speed screw in the reverse direction (clockwise) until the engine reaches its highest rpm; if the idle speed is higher than specifications, repeat Steps 2 and 5.
  10.  
  11. Continue to turn the idle mixture screw clockwise to obtain the specified rpm drop:
  12.  



4-speed - 40 rpm.
 
Hondamatic - 20 rpm.
 

  1. Replace the limiter cap and reconnect the cooling fan lines.
  2.  

Non-CVCC Civic - Hitachi 2-bbl (1974-77)
  1. Set the parking brake and block the front wheels.
  2.  
  3. Turn the headlights ON and the radiator cooling fan OFF . To make sure that the cooling fan is turned OFF while you are making your adjustments, disconnect the fan leads.
  4.  


CAUTION
DO NOT leave the cooling fan leads disconnected for any longer than necessary, as the engine may overheat.

  1. If equipped with a manual transaxle, position the shift selector in Neutral ; if equipped with a Hondamatic transaxle, position in shift selector in gear 1 .
  2.  
  3. From the carburetor, remove the plastic limiter cap from the idle mixture screw.
  4.  
  5. Using a tachometer, connect it to the engine with the positive lead connected to the distributor side (terminal) of the coil and the negative lead to a good ground. On 1976 models, disconnect the breather hose from the valve cover.
  6.  
  7. Start the engine. To adjust, turn the mixture screw (turn counterclockwise to richen), then, the idle speed screw for the best quality idle at 870 rpm (manual transaxle) or 770 rpm (automatic transaxle in gear).
  8.  
  9. Turn the idle mixture screw (clockwise), until the idle speed drops to 800 rpm (manual transaxle) or 750 rpm (automatic transaxle in gear).
  10.  
  11. Replace the limiter cap, connect the cooling fan and disconnect the tachometer.
  12.  

CVCC Models - Keihin 3-bbl (1977-79)
  1. Apply the parking brake and block the front wheels.
  2.  
  3. Start the engine and allow it to reach normal operating temperatures, the cooling fan should turn ON .
  4.  

If the fan does not turnON, load the electrical system (for purposes of adjusting the idle speed), by turning the high speed heater blowerONinstead. Do not have both the cooling fan and heater blower operating simultaneously, as this will load the engine too much and lower the idle speed abnormally.

  1. Turn the headlights ON . Position the transaxle's shift selector in Neutral (manual transaxle) or in 2nd (Hi) Gear (Hondamatic).
  2.  
  3. Using a tachometer, connect it to the engine with the positive lead connected to the distributor side (terminal) of the coil and the negative lead to a good ground.
  4.  
  5. Remove the plastic cap from the idle mixture screw. Start the engine and rotate the idle mixture screw counterclockwise (rich), until the highest rpm is achieved. Then, adjust the idle speed screw to 910 rpm (manual transaxle) or 810 rpm (automatic transaxle in 2nd gear).
  6.  
  7. Finally, lean out the idle mixture (turn mixture screw clockwise) until the idle speed drops to 700 rpm (manual transaxle) or 650 rpm (automatic transaxle in 2nd gear).
  8.  

If the idle speed cannot be adjusted properly, check and/or adjust the throttle cable.

  1. Replace the limiter cap (make sure the pointer is facing 180° away from the boss on the carburetor base) and disconnect the tachometer.
  2.  

Carbureted Engines (1980-83)

See Figure 1

Changes in the carburetors have made the adjustment of the idle mixture impossible without a propane enrichment system not available to the general public. The idle speed may be adjusted as follows:

  1. Operate the engine until normal operating temperatures are reached.
  2.  



Click image to see an enlarged view

Fig. Fig. 1: Common mixture adjustment plug on carburetor assembly

  1. Remove the vacuum hose from the intake air control diaphragm and clamp the hose end.
  2.  
  3. Using a tachometer, connect it to the engine.
  4.  
  5. With the headlights, heater blower, rear window defroster, cooling fan and air conditioner OFF , adjust the idle speed by turning the throttle stop screw to the rpm listed in the Tune-Up Specifications or underhood sticker.
  6.  

CVCC Models - Keihin 3-bbl (1980-82)

See Figures 2, 3 and 4

This procedure requires a propane enrichment kit, and, for California cars, a special tool for fuel/air mixture adjustment.

  1. Start the engine and warm it up to normal operating temperature. The cooling fan will come on.
  2.  
  3. Remove the vacuum tube from the intake air control diaphragm and plug the tube end.
  4.  



Click image to see an enlarged view

Fig. Fig. 2: Idle speed and mixture screws-Keihin 3 bbl carburetor



Click image to see an enlarged view

Fig. Fig. 3: Using special tool for mixture adjustment on California models



Click image to see an enlarged view

Fig. Fig. 4: Mixture adjustment using propane enrichment method

  1. Connect a tachometer.
  2.  
  3. Check the idle speed with the headlights, cooling fan and air conditioner OFF. Adjust the idle speed if necessary, by turning the throttle stop screw. Idle speed should be set to the specifications found in the Tune-Up Specifications Chart or on the emission control decal in the engine compartment.
  4.  
  5. Remove the air cleaner intake tube from the air duct on the radiator bulkhead.
  6.  
  7. Insert the tube of the propane enrichment kit into the intake tube about 4 inches.
  8.  
  9. With the engine idling, depress the push button on top of the propane device, then slowly open the propane control valve to obtain maximum engine speed. Engine speed should increase as the percentage of injected propane increases.
  10.  

Open the propane control valve slowly; a sudden burst of propane may stall the engine.

    1. If the engine speed does not increase, the mixture screw is improperly adjusted. Go to Step 8.
    2.  
    3. If the engine speed increase, go to Step 9.
    4.  


  1. Lean out the mixture until the idle speed (with propane on) increases. On California cars remove the screws on the right side of the carburetor then swing the tab out of the way. Insert the mixture adjuster all the way to the right and slip it onto mixture the screw. Turn the mixture screw clockwise to lean out the mixture as required.
  2.  

49 States models still have the normal mixture screw. No special tools are required for mixture adjustment.

    1. If the speed increase matched the specification, go to Step 11.
    2.  
    3. If the speed increase is out of specification, go to Step 9.
    4.  


  1. Check the speed increase against the specifications. If adjustment is required, adjust the engine speed to the propane enriched maximum rpm by turning the mixture screw. Turn it clockwise to increase; counterclockwise to decrease. Again, adjust the propane control valve for maximum engine speed.
  2.  
  3. Close the propane control valve and recheck the idle speed.
  4.  

Run the engine at 2500 rpm for 10 seconds to stabilize the condition.

    1. If the idle speed is as specified (Step 4) go to Step 12.
    2.  
    3. If the idle speed is not as specified (Step 4), go to Step 11.
    4.  


  1. Recheck the idle speed and, if necessary, adjust it by turning the throttle stop screw, then repeat Steps 7-10.
  2.  
  3. Remove the propane enrichment kit and reconnect the air cleaner intake tube on the radiator bulkhead.
  4.  
  5. On 49 States cars, install the limiter cap with the pointer 180° away from the boss on the carburetor body. On California cars, remove the mixture adjuster, then push the tab back into place and install the screw.
  6.  
  7. If the car is equipped with air conditioning, recheck the idle speed with the air conditioning on. The speed should still be within specifications. If the speed is outside the specification, remove the rubber cap on the idle boost diaphragm and adjust it by turning the adjusting screw.
  8.  

CVCC Models - Keihin 3-bbl (1983 Models)

This procedure requires a propane enrichment kit.

  1. Start the engine and warm it up to normal operating temperature. The cooling fan will come on.
  2.  
  3. Remove the vacuum hose from the intake air control diaphragm and clamp the hose end.
  4.  
  5. Connect a tachometer to the engine as per the manufacturer's instructions.
  6.  
  7. Check the idle speed with the headlights, heater blower, rear window defroster, cooling fan and the air conditioner OFF. Idle speed should be set to specifications according to the Tune-Up Specifications Chart or the emission control decal in the engine compartment.
  8.  
  9. Adjust the idle speed if necessary with the throttle stop screw.
    1. On the Prelude with automatic transmission, remove the frequency solenoid valve and the control valve. Disconnect the vacuum tubes and connect the lower hose to the air control valve.
    2.  
    3. On the Accord with automatic transmission, remove the air filter from the frequency solenoid valve and plug the opening in the solenoid valve. On all models, insert the tube of the propane enrichment kit into the air intake tube about 4 inches.
    4.  

  10.  
  11. With the engine idling, depress the push button on top of the propane device, then slowly open the propane control valve to obtain maximum engine speed. Engine speed should increase as the percentage of the propane injected goes up.
  12.  

Open the propane control valve slowly; a sudden burst of propane may stall the engine.

  1. Remove the air cleaner. Disconnect the vacuum hose from the fast idle unloader. Remove the bolts holding the throttle opener bracket to the rear edge of the carburetor.
  2.  
  3. On the Accord and Civic models, remove the carburetor nuts and washers. Remove the brake booster hose and throttle cable from their brackets. Lift the carburetor off the studs and tilt it backwards. Remove the throttle opener screw and bracket.
  4.  
  5. Remove the mixture adjusting screw cap from the throttle opener bracket. Reinstall the bracket. Using new O-rings on the insulator and new gaskets on the heat shield, install the carburetor.
  6.  
  7. Reconnect the vacuum hose to the fast idle unloader.
  8.  
  9. Install the air cleaner, start the engine and warm it to normal operating temperature. The cooling fan will come on.
  10.  
  11. Disconnect and plug the vacuum hose from the intake air control diaphragm.
  12.  
  13. On the Prelude models, label and disconnect all the lines from the carburetors. Disconnect the throttle cable and the vacuum hose from the throttle opener diaphragm. Disconnect the automatic choke lead. Drain the coolant and disconnect hoses. Remove the carburetors.
  14.  


CAUTION
When draining the coolant, keep in mind that cats and dogs are attracted by the ethylene glycol antifreeze, and are quite likely to drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantity. Always drain the coolant into a sealable container. Coolant should be reused unless it is contaminated or several years old.

  1. Place a drill stop on a 3mm drill bit, 3mm from end. Drill through the center of the mixture screw plug. Screw a 5mm sheet metal screw into the plug. Grab the screw head with a pair of pliers and remove the plug. Reinstall the carburetors in the reverse order of removal and refill the cooling system.
  2.  
  3. On all models, install the propane enrichment kit and recheck the maximum propane enriched rpm. If the enriched rpm is too low, lean out the mixture. If it is too high, enrich the mixture. Turn the mixture screw clockwise to increase rpm; counterclockwise to decrease rpm.
  4.  
  5. Run the engine for about 10 seconds to stabilize the mixture. Close the propane control valve and recheck the idle speed. Repeat the procedure until the idle rpm is correct. Remove the propane enrichment kit and reconnect the air cleaner intake tube.
  6.  

Keihin Dual Sidedraft (1983 Prelude)

This procedure requires a propane enrichment kit. Check that carburetors are synchronized properly before making idle speed and mixture inspection. It will also be necessary to remove the ECU fuse from the fuse box for at least 10 seconds to reset the control unit, after this procedure is complete.

  1. Start engine and warm up to normal operating temperature. Cooling fan will come on.
  2.  
  3. Remove vacuum hose from intake air control diaphragm and clamp hose end.
  4.  
  5. Connect tachometer. Check that the fast idle lever is not seated against the fast idle cam.
  6.  

If the fast idle lever is seated against the fast idle cam, it may be necessary to replace the left carburetor.

  1. Check idle speed with all accessories turned off. Idle speed should be 800 plus or minus 50 rpm (750 plus or minus 50 rpm on automatic transmission in gear). Adjust idle speed, if necessary, by turning throttle stop screw.
  2.  

If the idle speed is excessively high, check the throttle control.

  1. On automatic cars only, remove attaching bolt, then remove frequency solenoid valve and air control valve. Disconnect the two prong connector from the EACV and disconnect the hose from the vacuum hose manifold, then cap the hose end.
  2.  
  3. Disconnect vacuum tubes and connect lower hose to air control valve. Disconnect the vacuum hose from the air conditioning idle boost throttle controller. Disconnect the air cleaner intake tube from the air intake duct.
  4.  
  5. Insert propane enrichment hose into opening of intake tube about 4 inches.
  6.  

It is not necessary to disconnect intake tube. Opening for the tube is just behind right headlight. Check that propane bottle has adequate gas before beginning test.

  1. With engine idling, depress push button on top of propane device, then slowly open propane control valve to obtain maximum engine speed. Engine speed should increase as percentage of propane injected goes up.
  2.  

Open propane control valve slowly. Sudden burst of propane may stall engine.

  1. Propane enrichment maximum rpm:

    1983 manual transmission - 45 plus or minus 25 rpm.
     
    1983 automatic transmission - 110 plus or minus 25 rpm (in D3 or D4).
     

  2.  

  1. If engine speed does not increase per specification, remove carburetor.
  2.  
  3. Place a drill stop on a 1 / 8 " drill bit, then drill through center of mixture screw hole plug.
  4.  

If drilled deeper than this measurement, damage to mixture adjusting screw may result from bit. On the later models, remove the mixture adjusting screw hole caps, by pulling them straight out.

  1. Screw a 5mm sheet metal screw into hole plug.
  2.  
  3. Grab screw head with a pair of pliers and remove hole plug.
  4.  
  5. Reinstall carburetor.
  6.  
  7. Start engine and warm up to normal operating temperature. Cooling fan will come on.
  8.  
  9. Recheck maximum propane enriched rpm. If mixture is rich, turn both mixture screws 1 / 4 turn counterclockwise.
  10.  
  11. Close propane control valve.
  12.  
  13. Run engine at 2500 rpm for 1800 seconds to stabilize mixture conditions, then check idle speed. Adjust idle speed, if necessary.
  14.  
  15. Disconnect upper #22 vacuum hose from air leak solenoid valve at air jet controller stay, and plug end of hose, than connect a vacuum gauge to solenoid valve.
  16.  
  17. With engine idling, depress push button on top of propane device, then slowly open propane control valve and check vacuum. Vacuum should be available.
  18.  
  19. If no vacuum, inspect air leak solenoid valve.
  20.  
  21. Inspect thermovalve.
  22.  
  23. Remove propane enrichment kit and reconnect connector.
  24.  
  25. Install new plugs into idle mixture screw holes.
  26.  

Tailpipe Emission Inspection
CARBURETED MODELS
  1. Perform Steps 1-2 of the Propane Enrichment Procedure as outlined in this section. If necessary adjust the idle speed. On Accord and Civic models, disconnect the air cleaner intake tube from the air duct on the radiator bulkhead.
  2.  
  3. Warm up and calibrate the CO meter according to the manufacturer's instructions.
  4.  
  5. Check the idle CO with the headlights, heater blower, rear window defroster, cooling fan and the air conditioner off. The CO reading should be 0.1% maximum. If the CO level is correct go to Step 4. If the CO level is not correct, remove the idle mixture plug as described in the Propane Enrichment Procedure and adjust the mixture screws to obtain the proper CO meter reading.
  6.  
  7. Recheck the idle speed and adjust it if necessary by turning the throttle stop screw. Recheck the CO level and adjust, if necessary. On Prelude models, check and adjust the propane enriched rpm according the Propane Enrichment Procedure above.
  8.  

 
label.common.footer.alt.autozoneLogo