Air/Fuel Mixture: Adjustments
ADJUSTING IDLE - 911 SCAdjusting
Note: Make sure oil tank cap and seal fit properly before adjusting idle. Leaks through oil tank cap would cause incorrect readings.
Adjusting requirement:
Engine in perfect working condition and ignition timing set correctly.
1. Connect exhaust gas tap on test connection of catalytic converter.
2. Run engine to operating temperature (oil temperature about 90 °C/194 °F; oil temperature gauge needle at upper end of field outlined in white).
3. Connect CO tester according to instructions supplied with equipment.
4. Disconnect oxygen sensor plug in engine compartment on left side.
5. Turn control screw or bypass screw on throttle housing until specified engine speed is reached.
Specification: 900 ± 50 rpm
6. Check CO level. If CO level does not correspond with specified value, remove mixture control unit and remove threaded part of shear-off screw in access bore for mixture control screw.
7. Guide in Special Tool P 377.
8. Turning tool clockwise = richer mixture.
Turning tool counterclockwise = leaner mixture.
Note: The following points are important.
a) Always adjust CO level from lean to rich.
Example: If mixture is too rich, first turn idle control screw counterclockwise further than necessary and then clockwise to specified value.
b) Never exert force on special tool during adjustments (engine dies).
c) Turn control screw only very slightly, since even a slight turn will change CO content in exhaust gas considerably.
9. Remove special tool.
10. Accelerate engine briefly.
11. Wait until CO tester shows exhaust gas concentration at idle speed.
Specification: 0.6 ± 0.2 % CO @ 900 ± 50 RPM
12. Recheck idle speed and, if necessary, correct.
13. After finishing adjustments screw a new shear-off screw in access bore and tighten until screw shears off. Reconnect oxygen sensor plug.
14. Coat threads of cap nut for test connection on catalytic converter with Bosch VS 140 16 Ft or optimoly HT grease.