How Do I Change Spark Plugs on a Mariner Outboard Motor 75?

Your outboard's spark is as important as the one in your relationship.

Just as the spark of romance needs to be fanned occasionally, you have to check on your engine's spark plugs every three years or after 300 hours of motor operation. Like the small maintenance duties of romance, this inspection is to keep your outboard firing on all cylinders. With luck, you'll reach 300 hours before three years have passed, but whichever event occurs first, the plugs require inspection and, if necessary, replacement.

Things You'll Need

• Allen wrench
• Spark plug wrench
• Feeler gauge
• Anti-seize compound
• Torque wrench

Instructions

1. Move your right hand to the center-rear of the motor cover and press down on the latch. Tilt the cover forward, releasing the front edge of the cover. Set the cover on the ground.

2. Pull the wiring harness connectors out of the pencil coil connections. Remove the pencil coil connector mounting bolts with an Allen wrench and pull the pencil coil connector from the plug.

3. Use a spark plug wrench to remove the spark plugs. If the insulator is "rough, cracked, broken, blistered or fouled," the Mercury Outboard Care Guide recommends replacement.

4. Inspect the threads and electrode. Use a feeler gauge to ensure the gap on the electrode is 0.8 mm. If the threads that hold the plug in place are not corroded and if the electrode shows no evidence of damage from firing, wipe some anti-seize compound on the plug's threads and reinstall it.

5. Replace the pencil coils over the spark plugs with a twist-push motion and lock the coils in place with their retaining bolts.

Tips & Warnings

• The anti-seize compound on the threads is particularly important if you spend time boating in a saltwater environment.

• Torque the spark plugs to 20 foot-pounds. Torque the retaining bolts on the pencil coils over the plugs to 71 inch-pounds.

References

• Mercury Marine: Maintenance--Outboard Care