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Mikuni Turbo Carbs

Discussion in 'XJ Technical Chat' started by Fuller56, Mar 30, 2021.

  1. Fuller56

    Fuller56 Active Member

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    Looking for thoughts from the Collective. I have been e-conversing with Hogfiddles and Simmy about these carbs. They are from a Turbo that has only 2631 miles showing on it's odometer so that makes me think it has always had issues. The last fellow to own it spent A LOT of money trying to get these sorted without success. The initial symptom is the bike filling the sump with gas which leads one to believe the floats and needles are at fault which I agree with, especially with the carbs on the bench.
    Hogfiddles has sent me photos of the floats of an unmolested set of Turbo carbs and they agree with all the info I can find in manuals, videos and correspondence. Float height should be 17.5 mm and the physical appearance is that everything should appear level, or very close to it and the adjustment tab or tang should be parallel and level at this setting. Mine are neither. To get the float height at spec the tab is bent way up (carbs inverted). I can have one setting or the other. Attempting to by pass the measured float height and just wet set results in either the fuel level about 1/2 filling the bowl or running copiously out the carb throat. The float valves and seats have been replaced prior to it coming home with me and appear new. I am wondering if they are the correct parts for this model carb.
    Attached pics should show what I am dealing with.
    Thoughts anyone?
    @hogfiddles, @Simmy
     

    Attached Files:

  2. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    Where, on the float, are you measuring? Your “top line” is the crest of the round part of the float. First and last pic don’t look too far off..... middle two are are way low.

    dfox
     
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  3. Franz

    Franz Well-Known Member

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  4. Fuller56

    Fuller56 Active Member

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    Dave, measuring at the apex of the curve. The middle 2 measure out at about 22 mm as pictured.
    Franz, I believe the floats to original by the "patina". The PO reported he and his mechanic discussed replacing them but did not, they did replace the float needles and seats. I am wondering if they are the right parts, too long for application.
     
  5. Franz

    Franz Well-Known Member

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    I have been looking more into the floats, yes I think they are the right ones you have John, float needles and seats possibly incorrect.
     
  6. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    May very well be the wrong fuel valves. Pull one out and measure things, and then tomorrow evening I’ll pull one out and compare
     
  7. Simmy

    Simmy Well-Known Member

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    It could be some different float needles & bodies installed.
    I've attached a pic from the 2 types I've found installed in FZ600 carbs, they are both bottomed out.
    Bend the tab as needed, I don't see why they can't work.
    float needles.JPG
     
  8. Fuller56

    Fuller56 Active Member

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    Thank you Gentlemen. I will shortly go pull out a valve and measure it.
     
  9. Fuller56

    Fuller56 Active Member

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    The overall length of the valve seat is 20 mm including it's filter cap. From the rim of the opening to the shoulder that sits flush with the carb body is 8 mm.

    I could be over thinking this. I suppose I could just continue the wet set procedure and not worry about how it is supposed to look. But then again that has not worked on this bike in it's history.
     

    Attached Files:

  10. XJ550H

    XJ550H Well-Known Member Premium Member

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    are the valve bodies different lengths or are the needles shorter than each other.
     
  11. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    I will pull one apart tomorrow evening to compare

    dfox
     
  12. Fuller56

    Fuller56 Active Member

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    No, all are the same and all are new per the PO. Floats appear to be original.
     
  13. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    Floats look correct. I’ll open mine again tonight
     
  14. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    2778E54E-8F6C-49DE-B1D0-AB5F76DAE76E.jpeg


    3F622D44-3A64-475C-A9E0-CA0DAC9AAFC1.jpeg

    9C7157FC-BF90-4FCE-8499-4DA2CEB3CE91.jpeg

    75B749AB-26A3-4750-99B0-633947A0EDA6.jpeg
     
  15. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    Time for a follow-up:

    Fuller56 sent his rack up for me to look at and compare with one of my 650 turbo parts racks.
    The long and short of it is this:

    1. All his parts seem correct and match with my parts other than:

    a. The fuel valve needle has a slightly different shape at the plunger ( not enough to be of any significance)
    b. The float has an extra ridge/design on the side ( makes no difference); and the hole for the pivot pin has a bushing. ( again, makes no difference as the inside diameter is the same and there is no slop)

    So I ruled out any error with floats. The valves appear to be virtually identical, so those are ruled out, too.

    2. I believe the error is in the stated float height. After reading consistently that the float height is 17.5mm, yet it just doesn’t seem right, I searched for info on the carburetor itself. NOW, I started finding that with the bs30 on other bikes, the listed heights were 21.4mm or similar. Hmmmmmm....... is this another manual or something error? I looked to see if I have a turbo manual and I do not..... BUT— I came across a folder that has some service addendums in it, primarily for the xz550, and the xv920. The last few pages have an addendum for the 650 Turbo...... fairing updates, blah blah.....then I found one part for troubleshooting turbo/fuel/running issues. VOILA!!!! MAKE THE FLOATS ARE SET CORRECTLY. THE CORRECT FLOAT HEIGHT IS 21.5mm”. Read that again..... correction from Yamaha—— 21.5mm
    There we go. So I’m going to reset these for Fuller56 and we’ll go from there.

    Now the funny thing is the rest of the corrected measurement. It says in full “the correct float height is 21.5mm +5mm”

    WHAT????
    Does that mean it’s actually 26.5mm?
    Or should it say +/- 5mm?(range of 17.5-26.5mm???)
    Or should it say +/- .5mm? (My guess)

    so I’m getting a copy of this ready for LEN to have available.

    At any rate, that’s where things stand for the moment.

    Dave Fox
    -East Coast Parts Hoarder
    -Host, Central New York Carb Clinic
    -Registrar - XJ900RK Database (USA-Delivered)
    -XJ Police
     
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  16. Fuller56

    Fuller56 Active Member

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    Thanks Dave. He and I discussed this yesterday and I agree that this may be the root of 1 issue with this bike and may have lead to others. So I will be doing a more in depth deep dive into the fuel into crankcase, leaking issues. It appears this poor bike may never have been "right" straight out of the factory. The last 2 previous owners could not make it run and contain it's oil/gas correctly. We shall see. It appears I was correct in seeking knowledge from the Collective XJ Minds. Thanks again Guys.
    John
     
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  17. XJ550H

    XJ550H Well-Known Member Premium Member

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  18. XJ550H

    XJ550H Well-Known Member Premium Member

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  19. Simmy

    Simmy Well-Known Member

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    For now I wouldn't be too concerned what the dry float measurement is as long as there is room for the floats to close before they top out against the body.
    Beyond the carb floats leaking, the fuel check valve must also leak to fill the crankcase.
    Here is where I set my Turbo, not sure it's correct but it is running real good.
    fuel level1.JPG
     
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