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Idle Adustment Screw cross threaded

Discussion in 'XJ Technical Chat' started by BluesBass, Feb 15, 2012.

  1. BluesBass

    BluesBass Member

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    After tearing apart and rebuilding a carb rack, my #3 carburetor threads for the idle adjustment screw are cross threaded in the upper third of the bore. The lower 2/3 of the threads are in perfect shape, but I can't get the screw in straight.

    I've tried to carefully use the idle adjustment screw to re-cut the threads but after about a turn and a half the screw is clearly crooked and becomes VERY difficult to turn. I haven't made any attempt to force the screw - I know the alignment needs to be spot-on.

    So aside from replacing the carb body, what are my options to fix this problem?
     
  2. JeffK

    JeffK Well-Known Member

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    OK John, i'm not sure you arrived at the place you are but at least you're pretty sure of what's wrong which is pretty good considering it's very difficult to see the threads in there.

    Go to your local Harbor Freight and buy a set of picks...dental picks


    Now comes the tough part, let's assume for shits&grins that your rack is assembled....get two long 4x1's or similar and AFTER stuffing both ends of each carb with a NEW blue shop paper towel or equal, place a board on each side of the rack and lock it down in the vise. Make do if you can't do it that way....obviously, the carb needs to be clamped still.

    Now, go through the picks and find the little curled one. Gently put it down about halfway in a good set of threads and get a feel for it as you, while the top is held against the side, you rotate it "out" letting it run up the ramp of the thread......now move it to the bad one and do the same....GENTLY tap it with a very light rubber mallet when it sticks...GENTLY.

    Remember, it's better to practice twice and do it 4 times, gaining a little more each time then to rush and screw it up!!

    BRASS=SOFT WHICH = EASY TO TRASH!!
    Piece of cake.....

    jeff
     
  3. BluesBass

    BluesBass Member

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    Thanks for the response Jeff! I don't currently have the carbs assembled in the rack, just individual carbs fully assembled (minus the idle adjustment screw in the #3 carb for obvious reasons). I do have a few questions to clarify so I don't muck it up too badly.

    So did you mean a dental pick set like this one?

    So how do you mean with tapping it with a rubber mallet? Should I be tapping gently DOWN (into) the idle adjustment cavity, or attempt to tap it perpendicular to the cavity as to apply torque to cut the threads?
     
  4. TIMEtoRIDE

    TIMEtoRIDE Active Member

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    Chacal sells the thread tapping - chasing tool for that,

    Early 650's had a different thread pitch than later ones
     
  5. BluesBass

    BluesBass Member

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    Any idea exactly what year Yamaha changed the pitch?
     
  6. TIMEtoRIDE

    TIMEtoRIDE Active Member

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    First page of "The Catalog"

    a) Alloy steel, with coarse-pitch threads, used on 1980-81 XJ650 Maxim and Midnight Maxim model carbs. The best description I can give for "coarse" threads is that the threads are sized much like you would see on a standard screw (like the carb bowl drain screws, etc.).....the threads are rather "wide apart". This is in contrast to..........

    b) Alloy steel, with ultra-fine-pitch threads, used on 1982-up XJ650 models with Hitachi carbs, all XJ750 Maxim and Seca models, and possibly on early model XJ700 non-X Hitachi HSC33 series carbs. Ultra-fine thread pitch looks like screw threads used on some kind of jeweler's or watchmaker's incredibly-closely-spaced threads.

    The exact date for the "changeover" has been debated,
    Here are the taps - -

    HCP2398 THREAD CHASER & CLEAN-UP TAP, made from a high-speed uncoated steel. For use on all Hitachi idle mixture screw ports, for models using ultra-fine threaded idle mixture screws. NOTE: these threaded holes almost always need cleaning or re-tapping.
    $ 33.00

    HCP6335 THREAD CHASER & CLEAN-UP TAP, made from a high-speed uncoated steel. For use on all Hitachi idle mixture screw ports, for models using coarse threaded idle mixture screws. NOTE: these threaded holes almost always need cleaning or re-tapping.
    $ 24.00

    If you know what you are doing, you can make your own thread chasers, but they're never as good as "store-bought" ones.
     
  7. JeffK

    JeffK Well-Known Member

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    That's good TTR, very good info that I, and I'm sure lots others didn't know either.......Are you thinking the same thing I am....that he got a later screw for an earlier thread? or the other way....

    Blue- no,,this one is the set.


    http://www.harborfreight.com/6-piece-pi ... 93514.html

    ...and think about it, you sure don't want pound down on the thread....no, you just tap sideways while trying to hold the fulcrum and the point of contact in the thread in a straight line.....but def go with the kit if you can!

    Jeff
     
  8. day7a1

    day7a1 Member

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    Are we talking about the idle adjustment screw or the pilot mixture screws??? Me thinks there's some wrong word usage going on here, the fact that its on #3 doesn't help to cue from context...
     
  9. TIMEtoRIDE

    TIMEtoRIDE Active Member

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    umm - - quote,

    So aside from replacing the carb body, what are my options to fix this problem?


    The PILOT screw goes into the body of the carb,
    the IDLE screw goes into the lower bracket.
     
  10. RickCoMatic

    RickCoMatic Well-Known Member

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    Pilot Mixture Screw:

    Carefully run a M6X0.5 "Bottom Tap" down the length of the Pilot Mixture Screw Hole.
    >> You MUST use a BOTTOM Tap <<

    The Hole will be chased for Ultra Fine Thread.
    Use the right Pilot Mix Screw.
    Wrap the Screw's Threaded Upper with 2 layers of Teflon Tape.

    Idle Adjustment Rod and Rail Threads:

    Replace Rail and Rod.

    Remove Rail. Heat. Peen Threaded Hole on BOTH Sides, ... flattening-out and reducing the Diameter of Hole Drilled in Rail.
    Heat. Peen.
    Quench.
    Drill and Tap to Thread Size matching ROD End >> After << Cutting NEW Threads on Idle Adj Rod.

    Tap RAIL to Next Oversize.
    Machine a Large Cap Screw's End down to a Diameter that can have Threads matching the size and pitch tapped in the rail.

    New / New.
    Used / Used.
     
  11. BluesBass

    BluesBass Member

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    Sorry for the delayed response. It is the pilot mix screw. I also identified my problem: I was using a newer screw in an older carb. Once I identified the problem swapping out screws did the trick and everything went together beautifully. Got the carbs bench synched and floats adjusted, so now all that remains is valve clearance checks then vacuum sync and finally color tuned.

    Thanks forthe Input guys!
     

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