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Adjusting Idle Speed and Mixture

How do I adjust the idle speed and mixture?

Adjusting The Idle Speed And Mixture

On some float-type carburetors, you can adjust the air-fuel mixture and engine speed at idle. Check for an idle speed screw designed to keep the throttle plate from closing completely, and an idle mixture screw that limits the flow of fuel at idle. If your carburetor contains these screws, proceed below.

  1. With the engine off, remove the air filter and air cartridge.

  2. Locate the idle mixture screw and turn it clockwise until the needle lightly touches the seat. Then, turn the screw counterclockwise 1-1/2 turns.

  3. If your carburetor has a main jet adjustment screw at the base of the float bowl, turn the screw clockwise until you feel it just touch the seat inside the emulsion tube. Then, turn the screw counterclockwise 1 to 1-1/2 turns. Replace the air cleaner assembly and start the engine for final carburetor adjustments.

  4. Run the engine for five minutes at half throttle to bring it to its operating temperature. Then, turn the idle mixture screw slowly clockwise until the engine begins to slow. Turn the screw in the opposite direction until the engine again begins to slow (image A)
    Image A
    . Finally, turn the screw back to the midpoint.

  5. Using a tachometer to gauge engine speed (image B)
    Image B
    , set the idle speed screw to bring the engine to 1750 RPM for aluminum-cylinder engine or 1200 RPM for engine with a cast-iron cylinder sleeve.

  6. With the engine running at idle (image D)
    Image D
    , hold the throttle lever against the idle speed screw to bring the engine speed to "true idle." Then, repeat the idle mixture screw adjustments from Step 4 to fine-tune the mixture (image C)
    Image C
    .

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