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.
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