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"04 YZ450F Idles high occaisionally. Help?

Discussion in 'Fuel' started by MXracn746, Jul 29, 2013.

  1. MXracn746

    MXracn746 PR Addict

    Location:
    Obetz, Ohio
    Racing Number:
    746
    Anybody know why the thing would idle high only occasionally? Otherwise it runs good. No surging, and the hotstart does work.
     
  2. hershey

    hershey PR Elite

    Location:
    Ozone Layer
    Racing Number:
    01
    Try making the idle ckt just a tad richer. Turn out the fuel screw 1/4 turn.
    Check for any grit in the slide area ( the whole thing for that matter ) of the carb
    Check the bike for air leaks, ie... head gaskets and carb boots.
    Make sure the spark plug is tight

    Just a few suggestions
     
  3. MXracn746

    MXracn746 PR Addict

    Location:
    Obetz, Ohio
    Racing Number:
    746
    Thanks, I'll try it
     
  4. dwinship463

    dwinship463 PR Member

    Location:
    Elyria, OH
    those are good sugesstions, i would bet there is an air leak between the rubber manifold and cylinder head. the rubber gets old and shrinks and the clamps dont hold it tight anymore. best thing to do is replace the rubber. either replacing or not, remove the manifold and clean it and the cylinder head flange. put a small amount of yamabond 4 or similar sealer on the rubber and install back on the head. tighten clamp and let dry.do only the cyl head side, carb side should be clean and dry to install carb. the little spacer can be removed from the clamps so they can tighten down on an old manifold. then check idle and mixture adjustment. check oring on mixture screw first, if flat replace it. start at 2 turns out with mixture screw. turn screw in until the rpm goes up, note setting. then turn out till the idle drops some, note setting. put adjuster in the middle of the two settings or 1/2 turn in from where idle was dropping. also the yamahas for some reason get a lot of gunk in the upper part of carb above the slide. built up fuel residue or something. clean that out and make sure the rest of carb and passages are clear.
     
  5. Tom

    Tom PR Elite

    You also can spray wd40 while it is running around the boot and see if the idlel changes.
     
  6. baxterj787

    baxterj787 PR Addict

    Location:
    Xenia, OH
    +1 for Tom's suggestion on checking the manifold. WD40 will let you know right away if the manifold is leaking.

    Another quick, easy check is to unplug the TPS lead from the carb (white 3 pin Molex connector). When the potentiometer inside the TPS goes out, the ignition gets confused and will alter the ignition timing. Just unplug it and see if the problem goes away.

    If the manifold is OK, pull the slide out of the carb and check the seal between the front and back half of the slide (bubble number 12 on most online fiche finders). I had one go out on a 426 and it gave me fits trying to figure it out.
     
  7. BriarcliffMx

    BriarcliffMx PR Founding Father

    Also pull the needle out of the carb and make sure the clip is tight in the groove. Sometimes those get worn, and allow the needle to float and wiggle, which can make the carb idle sporadically after you return the throttle to idle.
     
  8. MXracn746

    MXracn746 PR Addict

    Location:
    Obetz, Ohio
    Racing Number:
    746
    Thanks guys I'll try them.
     
  9. MXracn746

    MXracn746 PR Addict

    Location:
    Obetz, Ohio
    Racing Number:
    746
     
  10. baxterj787

    baxterj787 PR Addict

    Location:
    Xenia, OH
    Yep, those o-rings are important! Glad to hear it is running properly.
     
  11. dwinship463

    dwinship463 PR Member

    Location:
    Elyria, OH
    sweet! glad i could help. ive seen the whole screw fall out of people's carb, or they loose the oring, washer or spring and just thread it back in and hope for the best.
     
  12. MXracn746

    MXracn746 PR Addict

    Location:
    Obetz, Ohio
    Racing Number:
    746
    It's still slightly erratic, but better. I'm going to try the seal between the slide halves next. I've already inspected and cleaned the rest. The needle clip is good. I have to order the seal. We'll see. thanks!
     
  13. MX724

    MX724 PR Addict

  14. dwinship463

    dwinship463 PR Member

    Location:
    Elyria, OH
    yes, thats it the seal between slide plate and throttle valve. if you guys need oring for carb rebuild on any bike with a kehin FCR carb by the igasket set from honda. the whole kit costs as much as one oring from yamaha or kawasaki! i dont think the seal beween the slide plate and throttle valve comes in that set though. but all the other orings, float bowl gasket, top gasket etc are included.
     
  15. MX724

    MX724 PR Addict

    That oring did the trick...crazy how big of a difference this had on how the bike runs.
     
  16. MXracn746

    MXracn746 PR Addict

    Location:
    Obetz, Ohio
    Racing Number:
    746
    Just replaced mine too. It did the trick.
     

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