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How to Set the Timing on a 35 HP Mercury Outboard

Updated February 21, 2017

When you tune up your outboard, part of the tune-up process is setting the motor's timing. The 35 HP Mercury outboard is equipped with a capacitor-discharge ignition system that's mechanically-advanced by link rods between the carburettor and CDI base plate. As the throttle opens, the base plate is rotated by the link rods to advance the timing. The process requires you have the motor in the water, running.

Ensure an adequate supply of cooling water to the motor. Press the cover latch on the back of the motor cover, tilt the cover forward and remove it from the motor.

Connect your timing light to the "Number 1" spark plug lead. Start the motor and shift it into forward. Allow the motor to warm up to its normal operating temperature.

Check the timing. Ideally, the timing at idle speed is 7 to 9 degrees Before Top Dead Center (BTDC). Disconnect the trigger link rod from the ball stud. Shorten the rod length to retard the timing greater than 9 degrees BTDC and extend the length to enhance timing less than 7 degrees BTDC.

Snap the rod back onto the ball stud and restart the motor. Cycle the throttle open and closed three times. Allow the motor speed to stabilise at idle and recheck the timing.

Open the throttle to the maximum rated engine speed, "wide open throttle" or WOT, and check the timing. Turn the engine off. If the timing requires adjustment, leave the throttle at WOT, loosen the jam nut on the WOT stop-screw and turn the screw counterclockwise until it does not contact the throttle bracket platform.

Turn the stop-screw clockwise until it makes minimal contact with the platform and turn it one full turn clockwise. Hold the screw with your screwdriver and tighten the jam nut. Restart the motor and check the timing for both idle and WOT.

Tip

The best ways to ensure an adequate supply of water is by setting the motor into a test tank deeply enough that its cooling water intake is below water level, or simply leave the motor on the boat and tie the boat securely to the dock.

Warning

Do not attempt this using a flushing attachment; the attachment won't provide enough water to cool the motor.

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About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.