How do I Adjust the Carburetor on a 40HP Evinrude?

Updated February 21, 2017

The adjustment on the carburettor on your 40-hp Evinrude is limited to adjusting the idle speed. The mixture is controlled by the electronic control unit, which also controls ignition timing. The adjustment involves disconnecting the idle air control valve and adjusting the idle bypass air screw. This adjustment fine tunes the amount of air that circumvents the idle air control valve, which is fully closed when the motor is warm, when your motor is running within its normal temperature range. This adjustment can be made at the dock.

Ensure that the boat is well-secured to the dock, put the emergency stop chip into place and start the motor in "Neutral," allowing it to warm up until the temperature gauge indicates it is at its normal temperature.

Push down on the motor cover lock and remove the cover. Clip the lead of a shop tachometer to the No. 1 spark plug. Start the motor and allow it to warm up to its normal operating temperature.

Read the tachometer. The idle does not require adjustment if the idle is between 750 and 850rpm. Locate the idle air control valve on the right side of the throttle body if the tachometer reading is above or below the 750 and 850rpm range.

Locate the hose that runs between the IAC valve and the silencer. Pull the hose from the silencer. Seal the end of the hose by placing a golf tee into its open end.

Locate the idle bypass air screw on the throttle body. Turn the idle bypass air screw clockwise with a screwdriver to increase the air flow and bring the idle to the proper level. Turn the idle bypass air screw counterclockwise to decrease the air flow and reduce the idle to the proper speed.

Speed the motor up to 2,000rpm, then slowly reduce its speed to idle. Allow it to idle for a few moments to stabilise, then check the idle speed again, adjusting it, if necessary. Remove the golf tee and reconnect the hose from the IAC valve to the silencer.

Things You'll Need

  • Shop tachometer
  • Golf tee
  • Screwdriver
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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.