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How to Adjust the Carburetor on a 1986 40HP Yamaha Outboard Motor

by Will Charpentier

At the beginning of boating season, listen to your motor for the subtle coughs and sudden hesitations that say something isn't well with the carburetor, the hub of the fuel system where fuel and air meet and mix then flow to the motor's combustion chambers, where, with the spark the ignition system provides, they power the motor. When the carburetor of your 1986 40-horsepower Yamaha outboard is properly adjusted, that power is smooth. Make the adjustments with the motor running and in gear: it's the perfect excuse to go boating.

Launch the boat and move to a study dock. Tie the boat off securely and shut the motor down. Remove the top cover from the motor.

Check all of the motor's electrical connections, including the battery, to make sure they're clean and tight. With the engine off and the gear shift in "Neutral," loosen the cam follower screw at the top of the throttle cam with a screwdriver. Back off the idle speed screw until the throttle plate, in the throat of the carburetor, closes.

Loosen the throttle cable jam nuts -- there are two -- located right below the throttle cam assembly, using a small adjustable wrench. With the throttle control at the idle position, move the cam follower roller against the throttle cam. Adjust the throttle cable sleeves in the mounting bracket so you center the roller on raised mark that's located on the throttle cam.

Allow a minimum "slack" of 1/16 inch and a maximum slack of 1/8 inch in the cable to keep the cable from binding. Test this by rocking the throttle cam from side to side, and measure the amount of throttle cam travel at the rod link ball with a machinist's ruler. Tighten the cable jam nuts and, with the the cam follower resting on the throttle cam -- it's the only thing that will -- tighten the cam follower screw that you previously loosened.

Move the throttle to the idle position, if it's not already there, and turn the idle speed screw clockwise in until there's a gap that's between 0.005 inch and 0.040 inch between the throttle cam and cam follower.

Start the motor and allow it to run at idle until it warms up to its normal temperature. Turn the idle speed screw on the throttle linkage to set the motor's idle at between 650 rpm and 700 rpm. Put the motor into "Forward" gear and turn the mixture screw clockwise until the engine starts to bog down and misfire. Back the screw out 1/4 turn, or more if necessary.

Items you will need

  • Screwdriver
  • Small adjustable wrench
  • Machinist's ruler

References

  • "Yamaha Outboard Repair, 2-250 hp 1-4 Cylinder, V4 and V6 Models 2- and 4-Stroke Models, Including Jet Drives"; Seloc Marine; 2007

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.