Idle mix screw threads are damaged

Discussion in 'Carb/Tuning' started by Eckhoff, Jul 29, 2019.

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  1. Jul 29, 2019 #1

    Eckhoff

    Eckhoff

    Eckhoff

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    The threads on the idle a/f mixture screw seems to be damaged. There is nothing wrong with the screw itself, the damage is on the carburetor.

    If I turn the idle a/f mixture screw all the way in, I can only turn it back out 2 rounds before it's completely loose. I'm supposed to turn it out about 2.5 rounds, but that can't be done now. This is on the rear right cylinder.

    Any suggestions on what I can do, except buy a new carb?
     
  2. Jul 29, 2019 #2

    one2dmax

    one2dmax

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    Possibly run a tap and clean up the threads? They are very fragile so you have to be careful. Or a good used carb (I MIGHT have a few available)
     
  3. Jul 30, 2019 #3

    Eckhoff

    Eckhoff

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    Do you know which thread tap I need for that?
     
  4. Jul 30, 2019 #4

    one2dmax

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    6mm x .5 if I recall.
     
  5. Jul 30, 2019 #5

    dannymax

    dannymax

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    Think you will find the threads were previously drilled out in an attempt to remove a frozen mixture screw.
    Most likely a new carb will be needed.
     
  6. Jul 30, 2019 #6

    Eckhoff

    Eckhoff

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    So it's not possible to carefully fill the hole with something kind of product, re drill it, and make new threads?
     
  7. Jul 30, 2019 #7

    CostalGrey

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    I’ve used jb weld in the past with really good success on dirt bikes and some other bits and pieces of a motor nature you need to coat the screw lightly with some sort of release agent
    Over coils may be an option but I’m not so sure you will match the screw thread
    New or used carb is likely the end result for you
    2 cents
    Eter
     
  8. Jul 30, 2019 #8

    dannymax

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    There are other passageways inside the mixture screw bore that would get plugged with product and those would be impossible to re-drill.
     
    sdt354 likes this.
  9. Jul 30, 2019 #9

    Stephan

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    Try getting an engineering business to make you a bush that has an external thread which you can drill and tap into the carb body, and the M6 x 0.5mm internal thread you need for the air screw.
     
  10. Jul 30, 2019 #10

    sdt354

    sdt354

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    You won't get better advice from anywhere than from Dann-o. He's our go to guy on carbs. He's saved me more than once. I might have a used body too. Repairs become expensive and not practical and will most likely be iffy as to if they'll work at all. Some of us have cut up a carb to trace the circuits Danny was talking about. Sorry to see you get bad news. Switching everything over is a breeze.
    Steve-o
     
  11. Jul 30, 2019 #11

    sdt354

    sdt354

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    Plus the possibility of future failure. Ruining a great riding experience.
     
  12. Jul 31, 2019 #12

    Eckhoff

    Eckhoff

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    I have been fixing this bike for 2 years now, basically everything is new and working good now, and finally got my license a month ago. And then these things happen. I can get a new carb, but then I will have to wait for several weeks again. How much would a used carb body cost me?

    I guess I have nothing to lose if I try to carefully drill the hole and fit a bush with M6 x 0.5 mm threads. Funny thing is it looks like the M6 x 0.5 mm tap costs about ten times as much as M6 x 1 mm..
     
  13. Jul 31, 2019 #13

    dannymax

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    The mixture screw tunnel is factory bored to 3 different id's with a chamfer for o-ring seating part way thru. You will need some machine tool experience plus an assortment of boring tools to accomplish what you propose.

    As mentioned previously, it appears someone before you has attempted to drill out a seized mixture screw, the chances are good that carburetor is already ruined.
     
  14. Jul 31, 2019 #14

    Eckhoff

    Eckhoff

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    Maybe a stupid question, but is the idle mix screw only doing something useful when the bike is idling? Because I do have some trouble with the idling, but when I ride it it seems to be working fine.
     
  15. Aug 1, 2019 #15

    dannymax

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    It is the air/fuel mixture screw. Your rough idle is probly due to plugged or partially plugged pilot fuel jets.
     
  16. Aug 17, 2019 #16

    Eckhoff

    Eckhoff

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    That’s probably the most plausible cause behind idle problems. But just in case anyone else reads this, my problem was actually a poor battery. I knew the battery was poor but didn’t really think it would affect the idle that much. Put a new battery in and the bike is so smooth on idle and take off, like new again.
     
  17. Aug 17, 2019 #17

    sdt354

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    The ide circuit is active more than just at idle. Off idle and into the lower midrange is effected.
     
  18. Aug 17, 2019 #18

    VMAXXIMUM

    VMAXXIMUM

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    If you had a poor battery, you should check your charging system before you run into the same problem.
     

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