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Bike revs on its own to around 3-4k rpm

Discussion in 'The Garage' started by OnePhate, Mar 24, 2012.

  1. OnePhate

    OnePhate XS400 Addict

    Hey, I finally got around to starting up the bike today. She starts perfect on the first turn off the starter with no choke (new plugs and filters) but after about 5 minutes at idle she decided she wanted to rev to 3k and 4k if I don't turn her off, again choke or no choke she does this even when riding. When started goes to about 2,500 for 5 seconds or less and then settles at 1,250 for a while.:wtf:

    TL;DR Will idle at 1,250 until warm then surges to 3-4k
  2. OnePhate

    OnePhate XS400 Addict

    I might have found the problem... I forgot to hook up the vacuum line to the tank, and just had the petcock set to "on"
  3. Pork Chop

    Pork Chop XS400 Junkie

    I think that would do it. :D
  4. drewpy

    drewpy Excess twin Top Contributor

    yep, as Charlie Chan says, " Sirry irriate"
  5. OnePhate

    OnePhate XS400 Addict

    Woops, seems it wasn't the problem after all! It persists again, I checked for leaks at the boots using starting fluid, and that didnt show up so i went a little more thorough with an unlit propane torch and couldnt find anything. Hmmm...
  6. why walk

    why walk XS400 Member

    When mine did that it was just because the throttle stop screw was too far in, depends what choke setting your using though.
  7. HoughMade

    HoughMade XS400 Guru Top Contributor

    What are your idle mixture screws set at? As you have surmised, that is characteristic of a lean condition.
  8. bentwrench

    bentwrench DOHC Crazy Top Contributor

    Add in that it happens after you've warmed up the engine, and you're likely looking at a leak that you'll only find when hot. Something is warming up and expanding, and then allowing a leak. It's likely not anything to do with the carbs (unless your carbs are getting hot, in which case you have other problems.)

    I would check things like head gasket, and valve tappet clearances. Anything you can think of that's getting hot, and then allowing a leak.

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