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Weird Idle Behavior

Discussion in 'The Garage' started by blabla, Apr 13, 2014.

  1. blabla

    blabla XS400 Enthusiast

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    MA
    So I've got a stock 1981 XS400, stock carbs (running cylinder air filters). My bike idles well once its warmed up, but it behaves very strangely sometimes. Once idling well, if I give it gas, the idle jumps up to about 3-4000RPM and stays there, and eventually returns after a few seconds. I figured this meant that the main jet might be too big.

    However, if I run the bike with the choke on, it runs perfectly. throttle response is great, and it idles well. I figured this meant that the bike was running lean, and cutting off the air was making it behave correctly.

    Does anybody have any suggestions about what to do in this situation? I'm lost
     
  2. bcware

    bcware Well-read Top Contributor

    Clean your pilot jets/circuit thoroughly and or your pilot jets are too small. If you have modified air filters/exhaust you're almost certainly just too lean; you need a larger pilot jet. You can also try adjusting your idle mixture screws.

    Another point of fine adjustment is the idle speed. This isn't likely the source of your problems but it can contribute to a finely tuned idle. It's something to keep in mind.
     
  3. xschris

    xschris a lifestyle not a trend Top Contributor

    After you do what bc says I would sync the carbs.
     
  4. blabla

    blabla XS400 Enthusiast

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    The carbs are perfectly clean, and idle adjustment is perfect. It idles great, like I said, its just that when I give it gas and get the RPM's anywhere above 3, it hangs there for 10-15 seconds then comes back down. sometimes it doesn't come back down at all.

    Would this be a small pilot jet?
     
  5. Sesty

    Sesty H.E. Pennypacker

    how many turns out are ur mixture screws?

    Like bc said hanging RPMs usually means running lean check for air leaks too (carb boots, not missing rubber washer on mixture screws)

    cant hurt to clean them again....
     
  6. blabla

    blabla XS400 Enthusiast

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    I'm not sure to be honest, I forgot after I set them. Somewhere around 2-3 turns out. So should I try a bigger pilot jet or main jet?
     
  7. xschris

    xschris a lifestyle not a trend Top Contributor

    If they are out of sync this will happen also.
     
  8. bcware

    bcware Well-read Top Contributor

    Yes. A rich mixture will tend to cause rpms to fall. A lean mixture will cause the rpms to hang.

    When yours rpms fall down to the 3k rpms range you're transitioning to pilot jet only.
     
  9. GraysonFleming

    GraysonFleming XS400 Enthusiast

    Vacuum leak perhaps? Had same issue with mine, found it was just a cracked hose that went from the petcock to the intake boots, caused irradic idle
     

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