How to Adjust the Idle on 35-HP Mercury Outboard

by Will Charpentier
Adjust the Idle on 35-HP Mercury Outboard

Adjust the Idle on 35-HP Mercury Outboard

Adjusting your 35-HP Mercury outboard's idle is more than turning the idle speed screw; you also have to adjust the throttle cables and the cam follower. The extended time required for this process means that a flushing attachment may not supply enough water to avoid engine damage. The 30- to 40-HP Mercury outboards all used some version of the Walbro WME carburetor, not all of which have a mixture-adjustment screw.

Turn the motor off and shift it into "Neutral." Use a standard screwdriver to loosen the cam-follower screw on the carburetor. Loosen the idle screw, located to the left of the throttle until the throttle plate is closed: the throttle-shutter positioner will not touch the taper of the idle speed screw. Loosen the jam nuts on the throttle-cable sleeves, just below the throttle cam.

Move the throttle to the "Idle" position. Move the throttle cable sleeves up or down in the mounting bracket until the cam-follower roller until is centered over the raised mark on the cam, and tighten the cable jam nuts.Tighten the cam-follower screw and turn the idle-speed screw inward until there is a gap of 0.005 inches to 0.04 inches between the cam and the cam follower.

Start the motor and allow it to idle until it reaches its normal operating temperature. Adjust the idle-speed screw so that the idle speed is between 700 and 800 RPM.

Items you will need

  • Screwdriver


  • A minimum of 1/16-inch slack is required to prevent throttle cables from binding when you reposition the throttle cables. A maximum of 1/8 inch is allowable. You can determine the amount of slack by measuring the amount of cam travel at the link ball rod by rocking the cam from side to side.


  • When you're working on your motor, disconnect the battery cable from the negative post of the battery by removing the nut from the battery post and lifting the cable from the post.
  • If you work on your outboard motor on your boat or in a tank, remove the propeller nut with a wrench and slide the thrust hub, propeller and washers from the propeller shaft. Failure to remove a propeller before operating an outboard out of the water during maintenance or preparation for long-term storage is an invitation to a propeller-strike injury, which can maim or kill.


  • "Mercury Mariner Outboards 2.5-275HP 1990-2000 Repair Manual 2 Stroke, Including Fuel Injection & Jet Drives--30/40 Hp 2-Cylinder Models"; Seloc; 2010

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.

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