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Rescuing mixture screw threads

Discussion in 'XJ Technical Chat' started by Minimutly, Oct 19, 2020.

  1. Minimutly

    Minimutly Well-Known Member

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    The mixture screws on my vmax are identical to the xj Mikuni ones, so hopefully some shared knowledge here.
    One of the screws had been drilled out, to the extent of taking the thread out of the carb body with it. So what to do - finding a single body in the UK is impossible. I thought I'd make a threaded insert - something like m6 inside/m8 outside. Sadly though, there isn't enough metal for this. So I've decided to try to make a pressed in straight insert, with m6 by .5 internal thread. I will report back, maybe with pictures...
    Anyone had any success with this?
     
  2. Jetfixer

    Jetfixer Well-Known Member

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    You might try to find a tap , of course it is probably fine thread .
     
  3. k-moe

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

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    Honestly, the best bet is a skilled welder who can fill the hole so a new bore can be drilled and tapped.
    A custom made insert (especially in the home shop) will have too thin of a wall to be secure.
     
  4. Polock

    Polock Well-Known Member

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    if you weld it you loose center, then toss it.
    i thought about that once but i was going to use a nylon plug
     
  5. k-moe

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

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    Center is measured before the welding is done, then the hole is bored and threaded in exactly the same way that it was done by the factory. It's not a home job, unless you have a home machinist's shop. At a minimum you need a good drill press and know how to make a fixture, even then I'd want a cross-slide table for the drill press.
     
  6. Minimutly

    Minimutly Well-Known Member

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    Maybe you can weld the xj Mikunis, but the vmax carbs are mazak, or some such pot metal - welding is not on...
    I can't see how it would be possible to weld either - the weld would need to go all the way down to the bore, and build up a new boss, to be machined totally - would take a very skilled welder not to see it all become a blob...
    I have done the calcs and ordered a tap. Sure, there isn't enough metal to tap inside the carb, but a pressed in insert could work. We'll see.
     
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  7. Minimutly

    Minimutly Well-Known Member

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    A couple of issues I see will be difficult to overcome - centering up on the idle screw hole accurately, and making sure the tapped hole in the new insert is concentric. Either will screw this up - but cocking the two up and aligning the errors could make it work. As if...
     
  8. Minimutly

    Minimutly Well-Known Member

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    Ordered m6 by .5 taps yesterday, allong with an m4 lh thread tap - better option than drilling in future I think?
     
  9. k-moe

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

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    Another route is to tap to the next largest size and make a new pilot needle.
     
  10. Minimutly

    Minimutly Well-Known Member

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    Now for some reason I this didn't really occur to me. I'll give it some thought....
     
  11. k-moe

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

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    I only thought of it after you mentioned taps. Then a lighbulb exploded.
     
  12. Polock

    Polock Well-Known Member

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    just in case, what body is it?
    it's nice to have a plan "B"
     
  13. Minimutly

    Minimutly Well-Known Member

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    It's no4, on my spare set. No4 on the set on my bike has the same issue, and my manual says they "may" be different sizes. So it's not like I'm walking if this fails, but I'd like to have a complete set to swap in, then try to fix the other set.
     
  14. Minimutly

    Minimutly Well-Known Member

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    IMG_2604.JPG
    Well I set up the carb on the cross slide of my Myford, used the Amolco milling attachment to drill it - it was really tight on height, had to cut down the drill to get it in, and even then had to open up the chuck all the way.
    I made an insert, had promised myself I'd make a dummy hole, and a dummy insert to test the fit, but heck, I went for it.
    The drilling went fine, I made the insert, allowed a bit for the drill cutting oversize, and had fully expected to be taping down the od to get it to fit. But poo, it's a sliding fit...
    Have to make another tomorrow night - will go for 3 thou oversize.
     
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  15. Franz

    Franz Well-Known Member

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    So you are making a press in insert the mixture screw will screw into. I would not know where start I have only used woodworking lathes.
     
  16. Minimutly

    Minimutly Well-Known Member

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    I have a Myford ml7 which I confess to never having used, a Colchester Triumph round top which I use a lot of. The Myford has a nifty milling attachment, it's simple but could be useful, but again, never used it untill today. The miller I use most of is a really old Frizt Werner, sort of post war, but not by much. It's way too heavy to be drilling carburretors, good for skimming engine blocks and heads, but my vmax was a bit of a challenge.
    I also have an engine boring machine and a delapena hone, bolted on top of my pillar drill...
    Sort of a home hobby workshop.
     
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  17. Franz

    Franz Well-Known Member

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    Great tools and skills to have. There is usually always a way to repair things with the right know how and machinery.
     
  18. Minimutly

    Minimutly Well-Known Member

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    I don't know about skills, I'm pretty much a self taught amateur.
    However, today I can reveal that the new, larger diameter (by 4 thou) was indeed a success. I parted it off at 9.8 mm long, put a skim of loctite on it and tapped it home. Testing with a used oring, washer and spring showed it came out into the carb throat, held tension to 5 turns out, so I hope it will have been a success. Won't be certain until I get to set the carbs up. I need to rebuild plus set float heights before I get to refit them.
    Now that I've done one, got the drills and taps, I think it could be done in a morning, so not a huge job really.
     
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  19. k-moe

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

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    Good work. And some poeple said that it can't be done (including me about the insert, but then I was thinking of a threaded insert, not a press-fit).
    Skills people: the more you have the more you can do. Being able to afford good tools helps though.
    My lathe is still in its crate from moving over a year ago. Perhaps I can make time this winter to wire the garage and set up shop. I picked up a milling attachment that I'm eager to play with.

    @Minimutly. What are the chances of you doing a writeup for the DYI section? Not many would be able to do the work themselves, but it might be useful to take along to a machine shop when farming the work out.
     
    Last edited: Nov 1, 2020
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  20. Minimutly

    Minimutly Well-Known Member

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    Yep, the threaded insert wouldn't do it - not enough metal as you say. Interference fit insert better though in one major way - less risk of misalignment.
    Put the "rack" back together today, need some bits to complete it, but I wanted it in one piece - I have too much stuff in bits around my workshop. Next job is to dip my 900 carbs in the ultrasonic bath, and they've been apart for months.

    I'll have a think about a write up - not sure where to place it?
     
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