Mercruiser Marine Engine Timing Procedures

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IMPORTANT: Failure to follow these timing procedure instructions will result in improper timing causing performance problems and possible severe damage to marine engines.

3.0L Models Only

1. Connect timing light (91-99379 or similar) to No. 1 spark plug wire. Connect power supply leads on light to 12 volt battery. Refer to Specifications – “Engine Rotation and Firing Order,” for cylinder numbering and location.

2. Connect a shop tachometer to engine.

NOTE: Before starting engine make sure the timing tab and mark on damper are clean. Chalk or white paint on timing mark on damper may help visibility.

3. Install a jumper wire between the two WHITE leads on the distributor. Use Quicksilver 91-818812A1, or fabricate a jumper wire using a 6 in. (150 mm) section of 16 gauge wire with two male bullet terminal ends connected.

4. Bypass the shift interrupt switch, as follows:

A  Disconnect wires at shift interrupt switch.

B  Temporarily join the engine harness wires together.

IMPORTANT: Do not fail to reconnect these two wires to the shift interrupt switch when timing procedures are complete.

5. Start engine and run at normal idle speed.

6. Aim timing light at timing tab, located on the timing gear cover and crankshaft torsional damper. Check the timing. Refer to “Specifications”.

7. Adjust timing by loosening distributor clamp and rotating distributor body as required until timing mark on damper or pulley lines up with the mark on tab specified in “Specifications.” Tighten clamp and recheck location of timing mark.

8. Aim timing light at timing tab and recheck location of timing mark. Repeat Step 7 until timing is correct.

9. Stop the engine. Torque distributor hold down bolt to 20 lb. ft. (27 N.m).

10. Reconnect the two wires to the shift interrupt switch.

IMPORTANT: Be sure to remove jumper wire before returning engine to service, otherwise timing will not advance.
11. Remove jumper wire at distributor white leads.

12. With timing light still connected, start the engine and run at IDLE. Verify that timing did advance to 12o BTDC, plus or minus 2o. At 2400-2800 RPM maximum (total) advance is obtained and should be 27o BTDC (plus or minus 2o).

13. Stop the engine and remove the timing light.

 

V6 and V8 Carbureted Models (Thunderbolt V Ignition)

1. Connect timing light (91-99379 or similar) to No. 1 spark plug wire. Connect power supply leads, if applicable, on light to 12 volt battery. Refer to Specifications – “Engine Rotation and Firing Order” for cylinder numbering and location.
2. Connect a shop tachometer to engine.
3. Using a jumper wire, connect the ignition system timing lead (PUR/WHT wire) to a good engine ground (-). This locks the ignition module into the “Base Timing Mode”.

NOTE: Before starting engine make sure the timing tab and marks on damper are clean. Chalk or white paint on timing marks may help visibility.

4. Start engine and run at normal idle speed. Allow engine to reach normal operating temperature.
5. Aim timing light at timing tab, located on the timing gear cover and crankshaft torsional damper.
6. If adjustment is required, adjust timing by loosening distributor clamp and rotating distributor body as required until timing mark on damper or pulley lines up with the mark on tab specified in “Specifications”. Tighten clamp and recheck location of timing mark.
7. Make sure that the distributor has been tightened. Remove the jumper wire from the timing terminal.
8. Remove jumper wire between the timing lead (PUR/WHT wire) and ground (-).

IMPORTANT: Be sure to disconnect the jumper wire from between the ignition system timing lead and ground (-) before attempting to resume normal operations. If the jumper wire is left in place, the ignition module will operate in the “Base Timing Mode”. This means that the additional timing advance features would not function.

9. Stop engine and remove timing light.

 

V6 and V8 EFI Models (EFI Ignition)

Two items of test equipment are required: an inductive pickup timing light and one of either a Scan Tool, Marine Diagnostic Code Tool or MerCruiser Special Timing Tool (Quicksilver 91-805747A1).
1. Connect timing light (91-99379 or similar) to No. 1 spark plug wire. Connect power supply leads, if applicable, on light to 12 volt battery. Refer to Specifications – “Engine Rotation and Firing Order,” for cylinder numbering and location.
2. Connect a shop tachometer to engine.

NOTE: Before starting engine make sure the timing tab and mark, or marks, on crankshaft damper are clean. Chalk or white paint on timing marks may help visibility.

NOTE: If NOT Using MerCruiser Timing Tool: With engine running, set the scan tool or Marine Diagnostic Code Tool to the SERVICE MODE and follow manufacturer’s instructions. On 350 Mag MPI Model, engine RPM will automatically increase to a higher RPM. You do not have to increase it manually, as stated in the following steps.

3. Connect the Timing Tool (or appropriate tool, as listed above) to the DLC connector (in the electrical box or on the electrical bracket) of the wiring harness.
4. Start the engine. Allow it to reach normal operating temperature.
5. On all models except 350 Mag MPI: Manually adjust the engine throttle to 1800 RPM.
6. Aim timing light at timing tab, located on the timing gear cover and crankshaft torsional damper. Check the timing. Refer to “Specifications.”
7. Loosen the distributor clamp bolt enough to just be able to rotate the distributor with some resistance.
8. Aim timing light at timing tab and recheck location of timing mark. Slowly rotate the distributor clockwise or counterclockwise to adjust the timing to specified degrees.
9. Aim timing light at timing tab and recheck location of timing mark. Repeat Step 8 until timing is correct. Torque distributor hold down bolt to 30 lb. ft. (40 N.m).
10. Manually close the throttle to bring engine RPM back to idle.

IMPORTANT: Be sure to disconnect MerCruiser Special Timing Tool from the DLC connector, or set the tool to NORMAL MODE if using the Scan Tool or Marine Diagnostic Code Tool, before attempting to resume normal operations. If the MerCruiser Special Timing Tool is left in pace or the setting of the tool remains in SERVICE MODE, the ignition system will operate only in the “Service Mode”. This means that the additional timing advance features would not function.

11. Disconnect the MerCruiser Special Timing Tool from the DLC connector. If using the Scan Tool or Marine Diagnostic Code Tool, set the tool to NORMAL MODE. Disconnect the tool.
12. Stop the engine. Turn ignition key to OFF position.
13. Remove timing light.