Air/Fuel Mixture: Adjustments

NOTE: Idle mixture adjustment is not required as a normal maintenance procedure and should be performed only if the carburetor has been disassembled or the vehicle fails emission testing. some states allow only licensed inspection stations to remove the concealment plug and perform any idle mixture adjustments.

Fig. 95 Drilling Idle Mixture Concealment Plug:

1. Remove the carburetor from engine. Mark all hoses to ensure proper reinstallation.
2. Secure the carburetor in a manner which will allow drilling a 1/8 in. hole through the base plate into the idle mixture concealment plug. Fig. 95
3. Using a small punch, drive the concealment plug out of the carburetor.
4. File smooth any burrs on the flange side of the carburetor and reinstall on the engine.
5. Connect timing light and tachometer.
6. Start the engine and confirm initial inspection conditions:

a. Engine coolant temperature is 185-205°F.
b. Lights, cooling fan and all accessories are OFF.
c. Transmission is in neutral or park.
d. Steering wheel is in straight-ahead position, (on vehicles with power steering).

7. Depress accelerator pedal once to release fast idle. Check idle speed and adjust to 800+/-100 rpm, if necessary.
8. Check the base ignition timing, and set to 7° BTDC +/- 2° if necessary.
9. Stop engine and disconnect the negative (-) battery terminal for at least 10 seconds. This will clear any diagnostic codes that might have been set.
10. Disconnect the oxygen sensor electrical connector.
11. Connect the battery, START the engine and operate at 2,000-3,000 rpm for 5-10 seconds. Return the engine to curb idle speed, and run for 2 minutes.
12. Using an exhaust gas analyzer, check the carbon monoxide (CO) level at the tail pipe.

CO Level: .1-.3 %

Fig. 3 Adjusting curb idle speed:

13. If not as specified, adjust CO level using the idle mixture adjustment screw, SAS-1 and the mixture adjusting screw (MAS).
NOTE: Do not touch the SAS 2 screw, it has been preset at the factory to maintain the relationship between the throttle valve and the free lever. If this setting is changed adjustment of the throttle opener and the dash pot cannot be done.
14. STOP the engine and reconnect the oxygen sensor.
15. START the engine and check the idle speed.
16. If no further adjustment is required, install a new idle mixture concealment plug.