Subject: Unstable Idle Speed Models: 1980-81 911SC
At lower ambient temperatures, engine idle speed can be unstable due to adjustment
-Adjust ignition timing at operating temperature to 5°BTDC (vacuum hose disconnected).
-Adjust idle RPM at operating temperature to 850 RPM.
-Adjust CO at idle to 0.6 ± 0.2%.
To avoid measuring errors, do not use ignition timing light with timing advance feature.
If above procedure does not stabilize idle speed:
-Remove deceleration valve
-Using a suitable socket, compress valve by 1 mm as shown.
-Reinstall the valve