How to Adjust Idle Mixture Screws on a 2-Cycle Outboard Motor

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Routinely adjusting the idle mixture screws on a 2-cycle outboard motor will keep the engine running efficiently and provide power when you need. The 2-cycle outboard motor has two idle adjustment screws on the top of the carburetor that can be adjusted with just a screwdriver and a vacuum gauge. You can fine-tune the idle mixture screws in a few minutes and keep the motor running strong.

Items you will need

  • Screwdriver

  • Vacuum gauge

Turn the idle adjustment screws clockwise with a screwdriver until you feel a slight resistance without over-tightening the screws. Turn both screws back counterclockwise 1-1/4 turns to set the starting position for the adjustment on the 2-cycle outboard motor.

Start the motor and let it warm up for about five minutes. Remove the rubber vacuum hose from the vacuum port and attach a vacuum gauge to the port.

Adjust the idle stop screw on the side of the carburetor near the choke cable, turning the screw with a screwdriver until the vacuum gauge is reading about 600 rpm.

Turn the idle mixture screw counterclockwise 1/4-turn at a time until you hear the engine begin to stall out, and turn it clockwise 1/4-turn.

Adjust the idle mixture screw by turning it clockwise 1/4-turn at a time until it starts to stall again and turn it back counterclockwise 1/4-turn.

Repeat Steps Four and Five for the idle air speed screw. Check the idle stop screw next to the choke and verify it is still running about 600 rpm.

Remove the vacuum gauge from the vacuum port, reattach the vacuum hose and shut off the 2-cycle outboard motor.


  • To avoid burns, be mindful of the hot engine parts while adjusting the outboard carburetor.
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