How to Adjust Idle on Mercury Outboard

How to Adjust Idle on Mercury Outboard 25hp 30hp 35hp 40hp 45hp 50hp 60hp

How to adjust idle on a Mercury outboard engine refers to the process of adjusting idle speed on a carburetor. Use the following information when making idle speed adjustments on your Mercury outboard engine. It should be noted, the engine must be completely warmed up before making idle speed adjustments. You cannot correctly adjust the idle in cold conditions.

Idle Speed Adjustment

How to Adjust Idle on Mercury Outboard

  1. With the Mercury outboard engine in neutral gear and in the water or test tank, start the engine and allow it to warm up.
  2. Shift the outboard engine into forward gear.
  3. Starting turning the idle adjustment screw on the carburetor to attain the specified idle rpm. Refer to the download repair manual for idle rpm specifications.
  4. Rev the outboard engine 2 or 3 times and then allow it to idle for 15 seconds. If the engine holds a steady and stable idle rpm then no other idle adjustments are needed. If the idle is unstable, then carburetor synchronization is necessary.

Always refer to the download factory service manual when performing work on an outboard. Additional idle adjustment procedures and critical specifications are listed, and often differs between year and model. 


  1. I have a 1998 90 hp Mercury outboard two-stroke. I can only get it started when the engine is in the high idle position but then it stalls about 2 minutes after.

  2. My engine (1992 200 hp) is having a fuel problem and I think it’s because the idle set too low. The motor idles @ 700 to 800 rpm’s, but when I put the engine in gear the rpm’s drop to 500 or 600. Then when I put it back in gear it will buck and kick and turn off unless I apply the throttle. Could it be the idle is set too low.

    • There are many things that can cause poor idling. I would start by checking if there is an air lead at the reed plate, base or carburetor. It can cause similar idling problems.

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