Air/Fuel Mixture: Adjustments

To conform with regulations, the idle mixture adjusting screw is adjusted and plugged with a steel plug by the manufacturer. Normally, this steel plug should NOT be removed.


1. Remove carburetor. Use tags to identify vacuum hoses for correct reinstallation. Cover intake manifold with a clean rag to prevent dirt from entering engine.
2. Remove mixture adjusting screw (MAS) plug.
a. Plug each carburetor vacuum port to prevent entry of steel particles when drilling.
b. Mark the center of the plug with a punch.

Fig. 147 Idle Mixture Adjustment Screw:

Fig. 148 Adjusting Idle Mixture:

c. Drill a 1/4 in. hole in the center of the plug, Fig. 147. As there is only 0.04 in. clearance between the plug and screw, drill carefully and slowly to avoid drilling onto the screw. The drill may force the plug off at this time.
d. Through the hole in the plug, fully screw in the mixture adjusting screw with a screw driver, Fig. 148. Be careful not to damage the screw tip by tightening the screw too tight!

Fig. 149 Removing MAS Plug:

e. Use a 9/32 in. drill to force the plug off. Fig. 149
3. Blow-off any steel particles with compressed air. Remove and inspect the mixture adjusting screw. If the drill has gnawed into the screw top or if the tapered position is damaged, replace the screw.

Fig. 151 Starting Idle Mixture Adjustment:

4. Screw in the mixture adjusting screw until the head is 0.138 in. below the lower surface of the carburetor, Fig. 151. Be careful not to damage the screw tip by tightening the screw too tight!
5. Reinstall the carburetor on the engine. Reconnect the vacuum hoses to the proper locations.
6. Reinstall air cleaner.
7. Start the engine and confirm preadjustment conditions:
a. Air cleaner installed.
b. Normal operating coolant temperature.
c. Choke fully open.
d. All accessories switched OFF.
e. All vacuum lines connected.
f. Ignition timing set correctly, (Refer to adjustment procedure).
g. Transmission in N range.

Fig. 152 Mixture Adjustment Screw:

Fig. 153 Idle Speed Adjustment Screw:

8. Attain MAXIMUM idle speed by adjusting mixture screw. Fig. 152
9. Set idle speed to 750 rpm M/T, 950 rpm A/T by adjusting idle speed screw. Fig. 153
10. Before moving to the next step, continue adjustments (8) and (9) until the MAXIMUM speed will not rise any further, no matter how much the IDLE MIXTURE SCREW is adjusted.
11. Set the final idle speed by screwing in the mixture screw clock-wise until idle speed drops to 700 rpm M/T, 900 rpm A/T. This is called the Lean Drop Method for setting idle speed and mixture.
12. Remove air cleaner and tap in new MAS plug until it is even with carburetor surface.
13. Reinstall air cleaner.
14. Check and adjust fast idle speed, (Refer to EMISSION CONTROLS for adjustment procedure).