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BS38 Mixture Screws

Discussion in 'The Garage' started by 79Standard, Dec 13, 2013.

  1. 79Standard

    79Standard XS650 Addict

    What are these adjusting- the amount of fuel or air, or both? I'm assuming fuel, so turned to the right = less fuel?
  2. angus67

    angus67 Welder's penetrate deeper!!

    adjust fuel. turning clockwise = less fuel
    Have you read the carb guide? grizld1 and 5twins spent alot of time writing that.
    This is a pretty basic question. its all covered in the guide.
  3. 5twins

    5twins XS650 Guru Top Contributor

    The CV carbs used on our bikes (both the BS38 and the BS34) have a true mixture regulating screw, NOT AN AIR SCREW as it is often erroneously called (even in the actual factory manuals). It regulates a portion (approximately 25%) of the fuel/air mix coming from the pilot jet. So yes, opening the screw makes the idle circuit richer, closing the screw leans it out. An air screw controls air flow to the pilot circuit so opening it adds more air and leans the mix.

    A way to determine which type the screw is on probably 99% of the carbs out there is by its location. If it's between the slide and the motor, it's a true mixture or fuel regulating screw. If it's between the slide and the air filter, it's an air screw.
  4. 79Standard

    79Standard XS650 Addict

    thank you both for the replies, much appreciated.

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