To adjust the idle speed and mixture, it is best to use an exhaust gas analyzer. This will insure that the proper level of emissions is maintained. However, if you do not have an exhaust gas analyzer, use the following procedure and eliminate those steps which pertain to the exhaust gas analyzer. When you have adjusted the carburetor it would be wise to have it checked with an exhaust gas analyzer.
- Leave the air cleaner installed.
- Run the engine at fast idle speed to stabilize the entire temperature.
- Make sure the choke plate is fully released.
- Connect a tachometer to the engine, following the manufacturer's instructions.
- Connect an exhaust gas analyzer and insert the probe as far into the tailpipe as possible.
- Check the ignition timing and set it to specification if necessary.
- If equipped with air conditioning, turn the air conditioner off.
- Put the transmission in Neutral. Make sure the hot idle compensator valve is fully seated.
- If equipped with a distributor vacuum control valve, place a clamp on the line between the valve and the intake manifold.
- Turn the engine idle speed adjusting screw in or out to adjust the idle speed to specification. If the carburetor is equipped with an electric solenoid throttle positioner, turn the solenoid adjusting screw in or out to obtain the specified rpm.
- Adjust the curb idle speed screw until it just touches the stop on the carburetor body. Back the curb idle speed screw out 1 full turn.
- Turn each idle mixture adjustment screw 1 / 16 turn richer (counterclockwise). Wait 10 seconds and observe the reading on the exhaust gas analyzer. Continue this procedure until the meter indicates a definite increase in the richness of the mixture.
This step is very important when using an exhaust gas analyzer. A carburetor that is set too lean will cause a false reading from the analyzer, indicating a rich mixture. Because of this, the carburetor must first be known to have a rich mixture to verify the reading on the analyzer.
- After verifying the reading on the meter, adjust the mixture screws to obtain an air-fuel ratio of 14.2. Turn the mixture screws clockwise (leaner) to raise the meter reading or counterclockwise (richer) to lower the meter reading.
- If the idle speed changes as the mixture screws are adjusted, adjust the speed to specification (see Step 10) and readjust the mixture so that the specified air/fuel ratio is maintained at the specified idle speed.
If the idle is rough, the screws may be adjusted independently provided that the 14.2 air/fuel ratio is maintained.
- Remove the analyzer, the tachometer, and the clamp on the vacuum line.