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Adjusting High Speed Mixture

How is the carburetor's high speed mixture adjusted?

Adjusting The High Speed Mixture

Some older carburetors contain a high speed mixture screw, near the throttle plate and opposite the idle speed screw. Under load, the high speed circuit increases air flow through the throat. Setting the high speed mixture involves running the engine until it is warm, stopping it to adjust the high speed mixture and then restarting for final adjustments.
  1. Run the engine for five minutes at half throttle to bring it to its operating temperature. Then, stop the engine.

  2. Locate the high speed mixture screw and turn it clockwise until the needle just touches the seat. Then turn the screw counterclockwise 1-1/4 to 1-1/2 turns.

  3. Restart the engine and set the throttle position to HIGH or FAST (image E)Adjusting the High Speed Mixture 79E
    Image E
    . Turn the high speed or main jet screw clockwise until the engine begins to slow. Then, turn the screw the other way until the engine begins to slow. Turn the screw back to the midpoint (image F)Adjusting the High Speed Mixture 79F
    Image F

  4. Once adjusted, check engine acceleration by moving the throttle from idle to fast. The engine should accelerate smoothly. If necessary, readjust mixture screws.

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