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'82 XJ550 mixture screw problem

Discussion in 'XJ Technical Chat' started by hedgehog, Jul 11, 2020.

  1. hedgehog

    hedgehog New Member

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    Hi Folks,
    I've got a 1982 XJ550R Seca, doing a carb clean and partial rebuild: new needle valves, gaskets, o-rings, and also replacing 2 of the starter plungers that were worn and 2 of the mixture screws which were stripped. I was able to get them out just fine, and Chacal hooked me up with the parts I need. But I am having trouble getting the new mixture screws in! I bought the HCP3745:
    HCP3745 OEM Mikuni IDLE MIXTURE SCREW SET....contains the brass idle mixture screw, spring, spring washer, and the o-ring. Original size for all XJ550, 1984-85 FJ600, 1986-88 FZ600, 1986-90 YX600 Radian, 1992-98 XJ600 Seca II, XJ650 Turbo, XJ700-X, XJ750-X, and XJ900RK, RL, N, FN, and F model carbs.

    I start screwing the mixture screw in place and it gets very tough to turn, well before it's supposed to. I thought it was perhaps the new spring that was adding resistance, so I took the o-ring/washer/spring out and tried to drive the mixture screw alone in there, just to see if it fit. Same result - it goes in a few turns and then slows down. I thought maybe the threads were messed up, but I went to one of the other carbs and swapped the mixture screw with that one. Same result - the new mixture screw doesn't want to go in, but the old one slides down without issue. I stared at the two side-by-side and they appear to be exactly the same, I can't tell by eye that anything is wrong (looking at lengths of the shaft/taper/needle, trying to gauge the diameters, don't have calipers but everything looks right).

    Am I missing something? Is it possible the mixture screws are wrong, even though they look right? Am I forgetting something? I thought about getting the tap to run it through the threads again. I see how that might help, but the fact that the old mixture screw goes in just fine makes me think the threads are not the problem.

    Any suggestions greatly appreciated!

    -tony
     
  2. k-moe

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

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    Are you positive that your machine has the correct carbs for that model?
     
  3. kosel

    kosel Active Member Premium Member

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    How certain are you that you got the old o-ring and washer out?
     
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  4. hedgehog

    hedgehog New Member

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    100% certain on this one.
    This is a great question. Can anyone point me to a resource that will help me ID my carbs? I thought I had found a thread on this topic a few weeks ago but I can't seem to find it right now.
     
  5. hogfiddles

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

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    Verify the threads in the body are the same as the threads on the new needle too
     
  6. hedgehog

    hedgehog New Member

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    The threads in the new and old mixture screws are an exact match - I can set them side-by-side and the threads mesh with each other. And as I drive the screw in, it doesn't feel like a cross-threading. I know that feeling (at my work we use an awful mix of english and metric hardware and I know the cross-threading feeling all too well). It feels tight, like there's some interference. And this feeling is there even if I leave the o-ring, washer, and spring out.


    I went at it again last night and just cleaned the hole again, used some lubricant on the o-ring, and more force than I am typically comfortable using, especially on brass. I managed to get one of the next mixture screws in with only the slightest damage to the head. Like I said, not my favorite to use that much force. But it went in all the way, I could see the needle tip of the mixture screw poking through just above the throttle butterfly, and then backed it off to match the 2 original ones (currently set at 2-1/2 turns out from bottom). I went to do the 4th and final mixture screw, prepared to use a bit of force. And I botched it - stripped the head. Drilled it out and removed with left-hand bit, and put in one of the old mixture screws (it is usable but the head was not in fantastic condition, so I thought I'd replace it while I was at it - nevermind I guess). So I suppose my problem is solved right now, but it's solved in a very unsatisfactory way. I plan to take the old and new mixture screws into work and going at them with our digital calipers to see if I can find a difference.

    Thanks for your suggestions, all. Like I said, my problem is "solved" for now, but I would love to know what the deal is.[/QUOTE]
     
  7. hedgehog

    hedgehog New Member

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    As to whether they are the stock carbs, I found this set of pics of a Mikuni BS28, and mine look exactly like the ones in the thread titled "
    MIKUNI CARBS, Part 1: EXPLODED VIEW, with pics"
     
  8. XJ550H

    XJ550H Well-Known Member Premium Member

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    dimple in carb hat about size of dime, no Nipples on the bowls to check fuel height ,brass butterflies


    it could be you need to run a bottom tap thread restoreer down the mixture screw holes(Cringe).
     
  9. Huntchuks

    Huntchuks Well-Known Member

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    Or a roll tap with the bottom ground off. That way no chips!
     
  10. Rooster53

    Rooster53 Well-Known Member Premium Member

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    It sure sounds like the new mixture screws are either damaged or a different pitch than the old. You probably need a microscope or high res camera (a good phone) to evaluate the two side by side.
     
  11. hogfiddles

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

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    Where did you get the screws from?
    Did you check to see if the tip is bent?
     
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  12. kosel

    kosel Active Member Premium Member

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    If you don't have a metric thread sizer, try your local hardware store. Near the small parts, they should have a board of size-labeled nuts and bolts. That will give the actual size and pitch of each. If they are different, you just need to source the right ones.
     
  13. JCH

    JCH Active Member

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    Hello Tony

    I'm sort of having the same issue,mixture screw very tight midway down while screwing in on a couple of carbs and tip of screw protrudes further out on throttle bore when bottom out,uneven,some mixture tips further out then others.
     
  14. XJ550H

    XJ550H Well-Known Member Premium Member

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    you would want to compare needles to see if they are the same or different.
    this could be a dirty or damaged thread in the carbs too.
     
  15. JCH

    JCH Active Member

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    I'm going to clean out the bores and see it that helps,they look a bit dirty,the screws are turning much better now after using anti size on them.
     
    Last edited: Jul 19, 2020

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