Mercury Outboard Idle Adjustment 6 8 9.9 15 HP

When a Mercury outboard motor runs slowly while disconnected from a load, it will typically idle smoothly. However, at some point the carburetor will fall out of tune and the engine will begin to idle erratically. Erratic idling can even cause the engine to stall. In which case, the carburetor will need to be adjusted to improve its running condition. Follow these rules to learn how to adjust the idle on a Mercury outboard motor.

Pre-carburetor adjustments

A carburetor is a device on an internal combustion outboard engine used for mixing air with a fine spray of gasoline. And before you begin to make adjustments to the carburetor, ensure the motor is ready for operation.


  1. Place the Mercury outboard in water.
  2. Check tiller handle cable adjustment for full throttle movement in both reverse gears and forward gears.
  3. If there is evidence of throttle movement slack, adjust jam nuts to eliminate and restore proper travel.

Idle speed screw (equipped models)

  1. Shift the Mercury outboard engine into neutral and back the throttle twist grip into slow position.
  2. Completely push in the primer/fast idle knob and then fully rotate the knob counterclockwise.
  3. Using a flat-head screwdriver, back idle speed screw off of cam follower.
  4. Turn the idle speed screw clockwise until it begins to make contact with cam follower, then stop. Then turn the idle speed screw clockwise an additional 1/2 turn so that it slightly opens the carburetor’s throttle plate.

Models not equipped with idle speed screw

  1. Loosen the cam locking screw.
  2. Push the cam follower down until it touches the throttle cam.
  3. Retighten the cam locking screw.

Low speed mixture screw

  1. Take off the access plug from the engine’s carburetor air intake cover.
  2. Slowly turn the low speed mixture screw clockwise until it lightly seats, then back the screw out 1-3/8 to 1-1/2 turns. Never turn the mixture screw in tight as this will damage the seat and needle.
  3. Do not reinstall the access plug at this time.

Idle wire adjustment

  1. Completely push in fast/primer knob and fully rotate it clockwise.
  2. Shift engine into neutral position.
  3. Check the fast idle speed by fully rotating the primer knob to clockwise position. Refer to the service manual for your model for fast idle speed RPM specifications. On most motors fast idle speed should be between 1500 – 2000 RPM.

Low speed mixture adjustment

  1. Start-up the Mercury outboard engine and allow it to warm up to normal operating temperatures (let it run for several minutes). Throttle the outboard motor back to idle for about 60 seconds to all the RPM to stabilize.
  2. Completely push in fast/primer knob and fully rotate it clockwise.
  3. Allow the engine to idle in forward gear, the turn low speed mixture screw counterclockwise until the Mercury outboard motor begins to load up from over-rich mixture.
  4. Slowly turn the low speed mixture screw clockwise until all cylinders are firing evenly and engine speed increases.
  5. Continue turning carburetor mixture screw clockwise until too lean a mixture is obtained and the Mercury outboard motor slows down and misfires.
  6. Now set the low speed mixture screw halfway between lean and rich.
  7. DO NOT adjust the carburetor leaner than necessary to attain reasonably smooth idling, this can cause internal engine damage. When in doubt, set mixture slightly richer rather than too lean.
  8. Reinstall the access plug into opening in carburetor air intake cover. 

Idle speed adjustment for models equipped with an idle screw

  1. Take the running engine out of natural and allow it to idle in forward gear. Make sure the fast/primer knob is completely pushed in and then fully rotate it counterclockwise to stop.
  2. Adjust idle speed screw to reach factory recommended idle speed.

Idle speed adjustment for models not equipped with an idle speed screw

  1. If the outboard motor is not equipped with an idle speed screw, then the carburetor has been pre-calibrated by the manufacture to maintain an idle speed of 650 ± 75 RPM in Forward gear, or as listed in your factor service manual.

It’s highly recommended to always use a service manual or repair manual when servicing or adjusting the idle on your Mercury outboard motor. Pictures, illustrations and important specifications will be listed in the manual.

3 thoughts on “Mercury Outboard Idle Adjustment 6 8 9.9 15 HP

  • January 23, 2017 at 11:33 pm

    What year and models to these instructions apply to?

    • January 23, 2017 at 11:33 pm

      This covers years, 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003. Horsepower models covered: Model 6 hp 6hp, Model 8 hp 8hp, Model 8 hp Sailmate Model 9.9 hp, Model 9.9hp Sailpower XR10/MAG10/Viking 10 hp, Model Sea Pro/Marathon 10hp, Model 15 hp, Model Sea Pro/Marathon 15hp.

  • January 3, 2021 at 7:33 pm

    I have a 1975 mercury 9.9. It runs good at home in a barrel in gear and in neutral put it in the river and put it in gear she goes dead. WTH


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