How to Adjust the Idle on Yamaha 9.9hp?


How do I alter the idle speed on my 9.9 hp Yamaha outboard motor? If feels like it idles too slowly when it is out of gear. Sometimes it stalls when I switch it into gear. So I wanted to increase the idle speed so the engine will increase RPM and not sputter out. And where is the idle speed screw located anyway?

I should also mention that the idle is a little rough. Even when I put it into gear but don’t put a load on it, the engine runs uneven and a little bumpy. I have not tried to go fast since it started acting up because I don’t want to break anything.

Any how-to advice or tips to get it running right? My Yamaha boat motor is a 2003 with model number 9.9MSHB. I would bring it to a mechanic but I’m a kid.

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Derek Shibuya 4 years 2020-12-02T02:17:30+00:00 4 Answers 5689 views Beginner 1

Answers ( 4 )


    Idle Speed Adjustment

    1. Start engine and allow it to reach normal operating temperature.
    2. Connect tachometer to the cylinder #1 high tension lead.
    3. Measure the idle speed to check if it’s out of adjustment. It should measure 750 ± 50 rpm.
    4. If out of specification, loosen the screw [1] of the carb throttle lever.
    How do I adjust idle screw on 9.9hp Yamaha outboard?
    5. Turn the pilot screw [2] inward until it lightly seats.
    6. Then turn the pilot screw outward 1-1/2 to 1-1/4 turn.
    7. Adjust throttle stop screw [3]. Inward increases idle speed and turning it out lowers it.
    8. Pull on the acceleration rod [4] until the fully retard screw makes contact with the fully retard stopper.
    9. Tighten the screw to 1 Nm (0.1 m•kg, 0.7 ft•lb).

    It’s recommended you adjust the throttle link before adjusting the idle speed.

    Best answer

    Rough idle or engine stalls can be numerous components at fault e.g. fuel filter, carburetor, low compression, reed valve, cylinder head gasket or ignition system. If engine stalls it could be the pilot screw setting, idle speed or fuel hose.

    Check the spark plug, if you haven’t already. Spark plug type should be NGK B7HS-10 or BR7HS-10. Gap the plug to 0.9 to 1.0 mm or 0.035 to 0.039 in. Torque plug specification is 25 Nm (2.5 m • kg 18 ft • lb).

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