How to Adjust the Idle on a Mercury 40-HP Motor

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Mercury is among the foremost brands of outboard boat motors. The Mercury 40 horsepower motor is a motor for medium to larger size boats. The Mercury 40 horsepower has dual fuel coolers to prevent vapor lock, which occurs when fuel temperatures are high enough so that fuel evaporates. The Mercury 40 horsepower motor requires minor maintenance such as water pump service from time to time. If the motor sputters or runs rough when idling, it requires adjustment.

Items you will need

  • screwdriver

Remove the motor housing cover from the top of your Mercury motor.

Start your Mercury 40 horsepower motor and allow it to idle.

Insert a screwdriver into the Mercury's idle adjustment screw, located at the bottom of the carburetor, and turn it to the left until the motor barely continues to run.

Insert the screwdriver into the valve screw and turn it in 1/8-turn increments to the right until the motor idles evenly and smooth.

Place the motor housing cover back on.


  • "Mercury Outboards, 3-4 Cylinders, 1965-1989 (Seloc Marine Tune-Up and Repair Manuals) "; Joan Coles; 1998

About the Author

Andrea Walk began writing in 2001, authoring user guides for technical software. She holds a Bachelor of Arts in political science from Loyola University, where she was a member of the Dean's Grade Review Committee for English. Walk is a member of the Automotive X-Prize team Global-E.