Idle Adjustment 20 hp Mercury 4-Stroke


How do I change the idle setting on a 20 hp Mercury 4-stroke? The engine runs too slowly when in neutral and stalls every now and then. The engine also shutters as its ticking over.

Can I make small alterations to the idle speed screw to achieve smooth idling? Are there instructions online that can show me step-by-step how to adjust the idle on a 2009 Mercury four-stroke 20hp? I am not even sure where the idle screw is located.

My motor looks like this…

How to Adjust Idle 2008 2009 Mercury 20hp

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Holly Yarger 3 years 2021-08-28T01:11:11+00:00 0 Answers 1282 views Beginner 1

Answers ( 2 )


    To properly make idle modifications make sure the throttle link rod is adjusted correctly, then start the engine and allow it to run for 2-3 minutes or until fully warm. Attach a simple tachometer to cylinder #1 spark plug lead and measure idle RPM while it’s out-of-gear. With the tach attached to the spark plug lead begin to adjust the idle screw until factory recommended idle RPM is achieved.

    Idle speed in neutral is factory set at 950 ± 50 RPM and 900 ± 50 RPM when in gear. This applies for both manual start and electric start 15 and 20 horsepower 2008 thru 2010 models. The idle adjustment screw location is pictured below.

    You can go here to download the complete factory repair manual for virtually any year and model outboard engine. The manual covers every aspect of service and repair—from a simple oil change to replacing a piston or water pump—covers it all.

    I have included a picture of idle adjustment procedures for your model. These adjustment procedures and specifications may differ depending on the year and model of your Mercury outboard.

    Idle Adjustment Instructions 15hp 20hp 2008 2009 2010 2011 2012

    Factory idle adjustment service procedure

    Best answer

    There’s more to just tweaking the idle screw to achieve a smooth running engine. A rough or  erratic idle can be an indicator of other problems such as a bad or improperly gapped spark plug, old gas, bad plug wires or dirty carburetor.

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