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Carburetor Problems BS34 1979 XS400

Discussion in 'The Garage' started by BlueBrick, Aug 10, 2022.

  1. BlueBrick

    BlueBrick XS400 New Member

    The Bike I have came with carbs that are not original and I am having several problems getting them set up.
    1. What is the correct level for the float bowl as measured with a clear tube? The original floats sank and have been replaced.
    • 2. The bike runs but is rather anemic. The jets and needle do not seem to be the correct sizes for the 400, but rather for a 360. Are the jets and needles just a simple changeout? IOW, are BS34 Carbs all the same? What differences? How do I tell? There are no specific markings on the carb.
    • What are the correct jets and needles for my bike?shows up
    I am having a rich condition which shows as the bike failing to maintain idle when warm.
  2. JPaganel

    JPaganel XS400 Addict

    2mm below the gasket sealing surface. Meaning, you are measuring off the carb body, not the gasket or the bowl.

    Carb bodies are the same for the same carb size. There are a couple of minor differences by year of production, but all jets and needles should fit the same.


    Jet, Main #132.5

    Jet, Pilot #42.5

    Undersized jets should make it lean. You might have too high float levels.
  3. BlueBrick

    BlueBrick XS400 New Member

    Thanks for the info! The float height was driving me crazy. The measure to the top of the float method just isn't all that accurate and I was running leaner than ideal when open.
    The rich condition on idle was due to conflicting information on the idle jet screws. I just took the bike on a 100 mile ride and it is working much better.
  4. tstidham

    tstidham XS400 Enthusiast

    One thing to keep in mind when adjusting the idle jet screws is the manuals spec for turns out is just a starting point. The actual setting will depend on your height above sea level. So set it to the factory spec initially, then adjust them in or out (usually out) to attain the highest idle speed for your elevation. As the idle increases however, you need to adjust the idle speed screw down to lower the idle speed. This keeps the carbs from transitioning onto the main circuit. So 1/4 turn on the idle jet screws to raise the idle, then turn the idle speed screw down to bring the idle back down. Rinse and repeat until the idle drops when turning the idle jet screw, then turn the idle jet back 1/4 to 1/2 turn. Set final idle speed to spec. I live near Philadelphia, and on mine, the bike ran best at 3.25 turns out.
    CaptChrome likes this.

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