Idle Speed Adjustment Suzuki Outboard DF 40-70 HP 1999-2011

Check idle speed using engine tachometer DF60 DF70

We often get the question, “how do I adjust the idle speed on my Suzuki outboard engine?”. This article discusses idle speed checking and adjustment procedures for the Suzuki DF 40 50 60 70 horsepower outboard engine. It’s highly recommended to always have a Suzuki outboard repair manual present before making adjustments to the boat motor’s idle speed. Critical specifications and factory adjustment procedures are listed in the manual.

Download Online Factory Service Repair manual PDF

IDLE SPEED 40-50HP

The following idle speed adjustment procedures apply to the 1999 thru 2011 Suzuki 40 thru 50 HP outboard engines (DF40 DF40T DF50 DF50T).

Bring the Suzuki outboard engine to normal operating temperature before checking idle speed. Make sure engine idle speed has stabilized before checking or making adjustments.

  1. Remove ignition coil bolt and remove No. 1 ignition coil.
  2. Connect H – T cord with plug cap adapter (special tool:  09930-88720) between spark plug and No. 1 ignition coil.
  3. Start the outboard engine.
  4. Connect engine tachometer (special tool: 09900-26006 engine tachometer) to the H – T cord with plug cap adapter (special tool 09930-88720).
  5. Check the outboard’s engine speed. Idle speed (no load) should be 800 – 900 r / min.

Idle Speed Adjustment

If the outboard engine idle speed is not within factory specification, use these step-by-step idle speed adjustment procedures. 

  1. Remove the idle air control IAC hose from the silencer case.
  2. Pinch the idle air control valve hose closed so as to stop the air flow from the idle air control passage.
  3. Adjust engine speed to 800 r / min. by rotating the by-pass airscrew. For reference, turning the airscrew in the clockwise direction will decrease engine speed whereas turning the airscrew in the counterclockwise direction will increase engine speed.
  4. Be sure to release the pinch from the idle air control valve hose so as to allow air to the idle air control valve. Then recheck the engine speed. Be sure the outboard engine idle speed has stabilized before checking. The engine speed should be 800 – 900 r / min.
  5. Ensure all parts removed are reinstalled.

NOTE: The outboard motor’s trolling/idle speed of 800 – 900 r/min is under the control of the idle air control system. If the outboard’s engine speed doesn’t return to the factory specifications listed, the idle air control or idle air control hose may be clogged, or the idle air control system could be malfunctioning.

IDLE SPEED 60-70HP

The idling speed adjustment procedures listed below are for the 2003 2004 2005 2006 2007 Suzuki 60 HP-70 HP outboard engine (DF60 DF60T DF70 DF70(W)T DF70WTH).

Check idle speed every 200 hours of use or every 12 months. Before checking or adjusting the idle speed, run the motor to normal operating temperatures. Make sure the engine’s idle stabilizes before making adjustments. Also, check the throttle valve and throttle link mechanism for smooth operation before making any adjustments to the idle speed.

  1. Start the Suzuki outboard engine and allow it to reach normal operating temperatures.
  2. Attach the motor tachometer to the ignition high-tension cord.
  3. Check the outboard motor’s engine speed. The idle speed in neutral gear should be 650 – 750 r/min

Idle Adjustment

If the outboard’s idle speed is not within the factory specifications listed above, follow these idle adjustment procedures.

  1. Ensure the closed throttle position (CTP) switch is in the ON position.
  2. Set the idle air control (IAC) valve duty. To set the idle air control valve duty to constant 22.5 percent, raise the outboard engine speed to 1000 r/min or higher by turning the by-pass air screw and hold that speed for 10 seconds. When following this procedure correctly, the buzzer will sound to alert that the idle air control duty is in the fixed mode.
  3. During fixed mode of the idle air control valve duty, adjust the outboard engine speed to 700 ± 50 r/min by turing the by-pass air screw.
  4. Open the outboard motor’s throttle valve to turn the closed throttle position switch off.
  5. You can manually cancel the fixed mode of the air control valve duty by turing the closed throttle position switch off.
  6. Close the throttle valve and reinspect the outboard engine speed. The engine idle speed should be stable at 650 – 750 r/min.

Special Notes:

  • Rotating the air screw in the counterclockwise direction will increase engine speed, whereas turing the air screw in the clockwise direction will decrease the engine speed.
  • When the idle air control valve duty is in the 22.5 percent fixed mode, the warning buzzer will sound with an interval of three seconds. After five minutes, the 22.5 percent fixed mode of the IAC valve duty will automatically be cancelled.
  • The outboard motor’s trolling/idle speed of 650 – 750 r/min is under the control of the idle air control system. If the outboard’s engine speed doesn’t return to the factory specifications listed, the idle air control or idle air control hose may be clogged, or the idle air control system is functioning incorrectly.

For more information regarding the idle air control system, refer to the idle air control system section in the factory repair manual. Suzuki 25hp repair manual.

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