How to Adjust Idle Suzuki 115hp (DF115)


How do I alter the engine idle to run faster or slowly while disconnected from a load? My 2003 Suzuki 115 hp outboard engine is running a little bumpy and irregular. The motor seems to run too slowly in neutral.

I suppose it runs too slow while out of gear and that is why it is running rough. I want modify the idle adjustment screw so that I can make it run smooth and balanced. How do I fine-tune the idle and where is the idle screw located? What is the stock idle rpm set at?

My boat motor is a 2004 Suzuki 115 hp 4-stroke 119.0 cubic inch 4-cylinder electronic fuel injection engine.

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Alonso Daniel 5 months 2020-12-15T19:06:00+00:00 1 Answer 196 views 1

Answer ( 1 )


    How to Adjust Idle

    Engine idle should be checked every 200 hours or 12 months, whichever comes first. Before you calibrate the idle speed you must bring the motor to normal operating temperature. You cannot properly check or adjust idle speed without warming the engine first. Also, be certain that the throttle valve and throttle link mechanism is operating smoothly.

    Most importantly, perform a complete tune-up before you adjust or check idle. Incorrect tappet clearance, worn or improperly gapped spark plugs and even dirty oil can cause an engine to idle unevenly.

    1. Start the Suzuki 115 horsepower engine and allow it to warm.
    2. Connect this tool to the high-tension cord/spark plug wire.
    3. Use the tool to check engine idle speed.

    4. In neutral gear the idle rpm should be 600 – 650 r/min.
    5. If idle speed is not in range, continue with instructions below.
    6. Make sure the closed throttle position switch CTP is in ON position.
    7. To set the idle air control valve IAC duty to constant 15%, increase engine speed to 1000 r/min., or higher by rotating the by-bass air screw and hold it their for 10 seconds.

    8. The caution buzzer should sound so as to notify you that the IAC duty is in fixed mode.
    9. Turning the screw counterclockwise will increase engine speed and clockwise will decrease speed.
    10. When the idle air control valve duty is fixed at 15%, the caution buzzer will sound for 0.5 seconds at 3 second intervals. It will continue sounding for 5 minutes until it automatically cancels.
    11. When in fixed mode of IAC valve duty, modify idle speed to 625 ± 25 r/min. by turning bypass air screw.
    12. Next open the warm-up lever to turn the closed throttle position switch off.
    13. For reference, the fixed mode of IAC valve duty can be manually cancelled by turning the CTP switch off.
    14. Close the warm-up lever and use the tachometer to recheck the engine idle speed. The idle rpm should be stable at 600 – 650 r/min.

    When the engine is in gear, the idle speed should be the same. If proper idle speed dues not return to normal make sure the IAC passage and the IAC house are not clogged.

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