Ignition Timing Adjustment

Procedure for setting and verifying ignition timing on the M127 inline-6 engine with mechanical fuel injection, including static and dynamic timing methods, and checking centrifugal and vacuum advance operation.

Warnings

Tools required

Parts

• Distributor gasket (if distributor must be removed) × 1 — Mercedes 000-158-79-80 or equivalent

Preparation

1. Ensure engine is at normal operating temperature (approximately 80°C/176°F) before beginning timing procedures, as cold engines will not reflect accurate advance characteristics.
2. Verify that ignition points gap is correctly set to 0.40mm (0.016 inches) and that points are in good condition without pitting or burning, as incorrect point gap will affect timing accuracy.
3. Check that distributor rotor and cap are in good condition with no carbon tracking, cracks, or corrosion, and that all spark plug wires are firmly seated.
4. Confirm that vacuum advance hose to distributor is in good condition without cracks or leaks, and that connection to intake manifold is secure.
5. Clean the crankshaft pulley and timing pointer area thoroughly so all timing marks are clearly visible, using degreaser if necessary.

Procedure

1. 1

   Locate and identify timing marks on crankshaft pulley
   The W111 M127 engine has timing marks cast into the crankshaft pulley at the front of the engine, visible from above on the driver's side. The pulley has multiple marks indicating TDC (Top Dead Center) and BTDC (Before Top Dead Center) positions. The TDC mark is typically the longest or most prominent line, with additional marks at 5°, 10°, and sometimes 15° BTDC positioned clockwise from TDC when viewing from the front. A stationary timing pointer is mounted to the timing cover directly above these marks. Use a flashlight and mirror if necessary to clearly see all marks. The crankshaft rotates clockwise when viewed from the front. If marks are faint or obscured, clean them thoroughly with a wire brush and highlight with white chalk or paint marker for visibility.
2. 2

   Locate cylinder #1 firing position
   On the M127 inline-6, cylinder #1 is at the front of the engine nearest the radiator. Remove all spark plugs to make hand rotation easier, or leave them in if using the starter motor method. Rotate the crankshaft clockwise using a socket and breaker bar on the crankshaft pulley bolt (27mm or 1-1/16 inch) until the TDC mark on the pulley aligns with the timing pointer. At this point, cylinder #1 or cylinder #6 will be at TDC. To determine which cylinder is on its compression stroke (firing position), remove the distributor cap and observe the rotor position. The rotor should be pointing toward the #1 cylinder terminal on the cap (marked or identifiable by tracing the spark plug wire). If the rotor points to #6, rotate the crankshaft one complete additional revolution (360°) until TDC mark aligns again and rotor points to #1. This is TDC #1 compression stroke, the reference point for all timing procedures.
3. 3

   Perform static timing check using test light method
   With engine off and ignition switch in the ON position, connect a 12-volt automotive test light between the distributor's low-tension terminal (the single wire connection on the side of the distributor body, not the coil wire) and a good engine ground. Loosen the distributor clamp bolt located at the base of the distributor body using a 13mm wrench—just loose enough that the distributor can be rotated by hand with moderate force but will not move on its own. Rotate the crankshaft backward (counterclockwise) approximately 20° from TDC, then slowly rotate it forward (clockwise) while observing the test light. Simultaneously rotate the distributor body slowly counterclockwise (retarding timing) until the test light just illuminates exactly when the timing mark corresponding to the base timing specification (typically 5° BTDC for the M127) aligns with the pointer. The light illuminates the instant the points open. This method establishes static timing and provides a baseline before dynamic verification. If the light does not illuminate at any point, verify ignition switch is on, test light is functional, and points are not stuck closed.
4. 4

   Verify base timing dynamically with timing light at idle
   Start the engine and allow it to reach normal operating temperature. Connect an inductive timing light to cylinder #1 spark plug wire (front cylinder) according to the timing light manufacturer's instructions, with the power leads connected to the battery. Locate and disconnect the vacuum advance hose from the distributor vacuum diaphragm unit (mounted on the side of the distributor body) and plug the hose end to prevent a vacuum leak that would affect idle quality. With engine idling at approximately 650-750 RPM (curb idle speed), aim the timing light at the crankshaft pulley timing marks. The specified base timing for the M127 is typically 5° BTDC, though some specifications call for 3-7° BTDC depending on model year and fuel octane availability. The mark corresponding to this specification should align with the timing pointer when illuminated by the strobe light. If adjustment is needed, loosen the distributor clamp bolt slightly and rotate the distributor body: counterclockwise advances timing (mark moves toward higher BTDC numbers), clockwise retards timing (mark moves toward TDC). Make small adjustments and recheck. Once correct, hold distributor firmly and tighten clamp bolt securely while rechecking that timing has not shifted during tightening.
5. 5

   Check centrifugal advance mechanism operation
   With the timing light still connected and vacuum hose still disconnected and plugged, gradually increase engine speed to approximately 3000 RPM while observing the timing marks through the timing light. The centrifugal advance mechanism inside the distributor should progressively advance timing as RPM increases. At 3000 RPM, total centrifugal advance should be approximately 20-25° beyond base timing, meaning the timing mark should show approximately 25-30° BTDC total (5° base plus 20-25° centrifugal advance). If the mark does not advance smoothly or fails to reach this specification, the centrifugal advance weights may be sticking due to corrosion or weak springs may need replacement. Allow engine to return to idle and verify timing returns to base specification, indicating the advance mechanism is not sticking in the advanced position. The advance should be smooth and progressive, not sudden or erratic. Document the total centrifugal advance observed for comparison with specifications.
6. 6

   Check vacuum advance operation and total advance
   With engine at idle and timing light still operating, reconnect the vacuum advance hose to the distributor vacuum diaphragm. Timing should immediately advance by approximately 10-15° as manifold vacuum acts on the diaphragm. If no advance occurs, check for vacuum leaks in the hose or test the diaphragm directly using a hand-operated vacuum pump. Disconnect the hose again and connect the vacuum pump directly to the vacuum advance diaphragm fitting. Apply 10-15 inches of mercury (in/Hg) vacuum while observing timing with the light at idle. Timing should advance smoothly by approximately 10-15° and hold steady as long as vacuum is maintained. If the diaphragm does not hold vacuum or timing does not advance, the vacuum advance unit has failed and must be replaced. Total maximum advance (base + centrifugal + vacuum) should be approximately 35-40° BTDC at 3000 RPM with full vacuum applied. This represents the complete advance curve. Excessive total advance can cause detonation, while insufficient advance causes poor performance and overheating.
7. 7

   Verify injection pump timing relationship
   While ignition timing is now correctly set, be aware that injection pump timing also critically affects performance on the M127 mechanical injection system. The injection pump timing should be verified separately using the injection pump timing procedure, but a quick functional check can be performed now. With the engine at normal operating temperature and idling smoothly at 650-750 RPM, listen for smooth, even running without misfiring or hesitation. The engine should accelerate cleanly without stumbling when throttle is opened. If ignition timing is correct but engine runs poorly, exhibits black smoke, or lacks power, injection pump timing may be incorrect. The injection pump on the M127 has its own timing marks on the pump drive coupling, and timing is typically set to begin injection at approximately 24° BTDC for cylinder #1. Both ignition and injection timing must be coordinated for optimal performance. If either is significantly incorrect, the engine will run poorly despite the other being correct. Note any performance issues for further diagnosis of injection system timing.

Reassembly

1. Ensure distributor clamp bolt is securely tightened after all adjustments are complete, typically to approximately 15-20 Nm (11-15 ft-lbs), firm but not excessive to avoid cracking the distributor body.
2. Reconnect vacuum advance hose to distributor and verify connection is secure without cracks or leaks in the hose.
3. Reinstall distributor cap and verify all spark plug wires are firmly seated in correct firing order: 1-5-3-6-2-4 for the M127 inline-6.
4. If spark plugs were removed for easier rotation, reinstall and torque to specification (approximately 25-30 Nm or 18-22 ft-lbs).
5. Remove timing light connections and verify all tools and materials are removed from engine bay before final test drive.

Verification

• Start engine and verify smooth idle at approximately 650-750 RPM with no misfiring, stumbling, or irregular running that would indicate timing or ignition system problems.
• Perform a final timing light check at idle with vacuum disconnected to confirm base timing remains at specification and has not shifted after tightening distributor clamp.
• Test drive vehicle and verify smooth acceleration without detonation (pinging or knocking sounds under load), which would indicate excessive advance, and without hesitation or poor power, which would indicate insufficient advance or injection timing problems.
• Monitor engine temperature during and after test drive to ensure engine does not overheat, which could indicate timing is too retarded or other cooling system issues.
• After test drive and cooldown, recheck base timing one final time to ensure distributor has not shifted due to vibration or inadequate clamping force.
• Document final timing settings including base timing, centrifugal advance at 3000 RPM, and vacuum advance specification for future reference and troubleshooting.

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