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Idle mix screws: the difference between coarse and fine thread

Discussion in 'XJ Technical Chat' started by jamiedransfield, Jan 21, 2016.

  1. jamiedransfield

    jamiedransfield Member

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    So ive been bumbling and learning my way though my xj650 fitted with the hitachi hsc32 carbs *shudder*

    One of the things i wasnt sure of was, until today, was if i had coarse or fine threaded idle mix screws.
    As ive yet to see a directly comparable photo I had so far assumed mine were fine.

    As it happens, i just aquired a spare set of junk/learning carbs.
    What i found was that these junk carbs were finer than the ones on my bike!

    So i was setting my baseline a good 1 turn out further than factory base spec, causing a rich mixture, and fouling the plugs.

    For all those interested, or unsure about your idle mix screws, heres a side by side photo so you can tell the difference for yourself!

    Note aside from the different thread, the very differently shaped tips.

    Anyways hope someone may find this useful.

    Niw back to figuring out why im still running so damn rich...
     

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  2. XJ550H

    XJ550H Well-Known Member Premium Member

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    course or fine is a reference to the thread pitch in metric it is mm per thread

    so a M4-.7 X14 bolt 4 mm diameter .7 mm between threads or 10 threads in 7 mm


    still 1 turn out over base line?
     
  3. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    The "working" part of the needle tips are really the same shape (the entire needle is the basically same shape between the coarse and fine-thread versions for the HSC carbs), the needle shown on the right (coarse thread version) has the tip of the needle broken off, that's why they appear different.

    Initial settings are as follows:

    Coarse threaded screws: 1.5 - 2.0 turn out from gentle bottoming of the screw.
    Fine threaded screws: 2.5 - 3.0 turn out from gentle bottoming of the screw.
     
  4. jamiedransfield

    jamiedransfield Member

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    I didnt think the tip was broken off...? May just be the positioning for the photo.
    Ill check with a magnifying glass and confirm tonight tho.

    The main reason for this post was to show what both coarse and fine threads look like.

    I initially thought mine were fine because i didnt have anything to compare it to.
    I thought other members may be interested to see the difference side by side.
     
  5. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    Yes, upon a closer look it may just be the angle of the photo. It's just that the bottom of the needle tip (on the right) looked "uneven" which appeared to be that a small section had fractured off.

    It's a great picture to illustrate the differences.
     
  6. hogfiddles

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

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    I will disagree.... I don't believe the tip is broken off.
     
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  7. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Check the transitions of the needles. The one on the right is not the same as the one on the left.

    Are you positive that you have a spare set of Hitachi HSC32 carbs?
     
  8. jamiedransfield

    jamiedransfield Member

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    I sure do.
    Just ones the old coarse type and the other is the newer fine type
     
  9. tabaka45

    tabaka45 Well-Known Member

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    Interesting. I think I have the fine thread, but I'm still set at less than 2 turns. If I had a choice I would prefer the course threads since they would probably be harder to strip. I wonder if Hitachi started with the fine threads and then went to the course threads for that reason?
     
  10. XJ550H

    XJ550H Well-Known Member Premium Member

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    there is a trade off for the course threads in the fine tuning of the air/fuel mixture
    the course threads do cut deeper into the metal so more support
     
    Last edited: Jan 23, 2016
  11. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    Nope, they started with coarse threads and then went to the fine threaded versions.
     

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