Air/Fuel Mixture: Adjustments

Duty Cycle Graph:

Idle Mixture Adjustment:

Perform idle mixture adjustment only after carburetor was removed from engine or if carburetor required disassembly. Perform adjustment on both main and slow duty solenoid valves using a suitable dwell meter. Set dwell meter range to four cycle (four cylinder) and adjust idle adjusting screw so that needle indicates a deflection equal to the following formula: Dwell(degrees) = 90/100 X Duty(%).

1. Connect a suitable tachometer and dwell meter.
2. Warm up engine to normal operating temperature.
3. Check dwell meter needle swing. If idle speed is satisfactory and duty ratio is as specified in the duty cycle graph (controlled, dwell meter needle steady), idle mixture is satisfactory.
4. If dwell meter needle does not fluctuate normally (erratic), increase and maintain engine speed to between 2000 and 3000 rpm for approximately 2-3 minutes, then return to idle speed. If dwell meter needle swing is normal (controlled duty ratio), check idle speed and mixture (duty ratio). If idle speed and mixture (duty ratio) are incorrect, check the following components:
a. Ensure ignition timing is adjusted to specifications.
b. Check for carburetor contamination.
c. Check vacuum hoses for condition and proper connection.
d. Check condition of spark plugs.
e. Check air filter element for obstruction.
5. Repair components as necessary, then check idle speed and duty ratio.
6. Check dwell meter swing. If dwell meter needle continues to fluctuate erratically, refer to Computers and Control Systems section for further troubleshooting procedures.
7. If dwell meter swing is normal, adjust duty ratio and idle speed to specifications by removing concealment plug using a suitable drill, then rotating adjustment screw. If adjustment for duty ratio and idle speed cannot be obtained, refer to Computers and Control Systems section for further troubleshooting procedures.
8. If dwell meter swing is normal and duty ratio and idle speed are satisfactory, road test vehicle for approximately 10 minutes, then recheck adjustments. If duty ratio and idle speed are as specified, idle mixture is satisfactory.