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Thread: Mixture screw threads trashed

  1. #1
    t3rmin Guest

    Default Mixture screw threads trashed

    I've discovered the threads on my #3 carb's mixture screw (the one with the needle-like point) are trashed. Dunno what the PO did but it screws in/out *really* hard and visually the threads are flattened.

    Any ideas on how to best resolve this? My thoughts so far involve finding a screw at the hardware store with the same threads and running it into the carb to try to clean up the threads inside, then finding a nut that fits the screw and running it through to clean up its threads. The threads are awful fine so hopefully I can locate a screw and nut that match.

    If I can't recover this screw, does anyone know where I can buy them or does anyone have an extra one around? I'm also missing two washers and one spring from the other mixture screws and haven't had any luck finding replacements either. None of the microfiches list these parts since I guess the mixture screw adjustment was set at the factory and not considered user serviceable...

  2. #2
    Join Date
    Sep 2005


    The screws, washer, spring and o-ring come in the carb rebuild kits. You may be able to find the washers and o-rings separately. I think I have a spare screw from a rebuild kit I never used (PM me). You may be able to find a similarly threaded long steel screw and tap it out but, IMHO, your best bet may be to find someone parting out a rack of carbs from a similar bike (some models share carb bodies and you can just replace the jets). I had a similar thing happen to me with my #2 on the 550. Even when I managed to tap out the threads using a similarly sized steel screw, the carb wouldn't work right (I couldn't set the idle mix properly on it). I eventually found a #2 on this web site and its been working great since.

  3. #3
    t3rmin Guest


    Well that is the same carb off of which I broke a float post (but fixed rather nicely with JB weld), so it wouldn't be a bad idea to replace it. But as usual I'm trying to get by on the cheap. From a practical standpoint, I don't see why I can't make this carb work if I can fix the mixture screw, provided I can get all the metal shavings cleaned out from the cross-threading.

    Actually I have seen the screws in rebuild kits, but as I'd already ordered a full o-ring set and bowl gaskets separately, I didn't particularly want to shell out for the kit just for the screw. I'll PM you about that screw, since the head on this one is starting to look questionable anyway and I'd hate to get it stuck.

  4. #4
    t3rmin Guest

    Exclamation Help! Can't find a tap screw!

    Well I've been to the hardware store and they don't have anything with the right threads to tap out that socket. Anyone have any idea what the thread pitch is on those screws (it's an M6 diameter) and where I can find (online or otherwise) something to tap out the hole?

  5. #5
    chiphead Guest


    If it was me, I'd post it on here and look for a carb body. I found one through an excellent member here and I'd be willing to bet you may too. The passages that feed that pilot fuel screw have extremely small holes and I think you're just asking for trouble.
    My 2 cents.

  6. #6
    t3rmin Guest


    I'm sure you're absolutely right and that's probably very good advice... ;-)

    I'm going to have one more go at it, though. After MUCH searching I did manage to find a an M6-0.50mm tap (ordered an M6-0.75mm too, just in case my measurement was off) at and DimitriT has graciously offered a spare mixture screw.

  7. #7
    Cyg Guest


    are you talking about the air screw that is on the bottom of the bowls and is exposed and facotory set? if so I talked with a mukuni dealer and he said that those parts are hard too find oem since we are not suppose too touch those. I had the same exact thing happen too my gs 750 (well my sons). The guy turned em in so hard he broke a piece of brass off inside the carb body. I took a fine wire brush filament and used that too poke it thru then shot air thru them after using pb blaser on it for two nights. It did work free and shot right out.
    But I had a problem with the rubber washer. sealing up the gas. I asked the mukuni dealer if he had any and he said they don't usally carry that part. So I said what if I grind down the screw too a point again, turn it in the recommended amount of turns set by the factory and then iron/ jbweld them permantly shut? He said too do it. since once they are set the right amount they should never be bothered with again. So I did it like he recommneded or allowed. Today the bike has not dripped any gas, she runs on all four cyclinders and it will go anywhere.
    So maybe this fix will help you out. dont know, you can only try.

  8. #8
    t3rmin Guest


    Quote Originally Posted by Cyg View Post
    are you talking about the air screw that is on the bottom of the bowls and is exposed and facotory set?
    I think that's how it's set up on older carbs. On mine the only external adjustment besides the throttle is these screws. They're called the mixture screws and they're on the top, toward the motor side. I think they are factory set, though, and not considered user serviceable. I assume that's why none of the microfiches ever list 'em.

    Fortunately none of the tips are broken off so I won't have to deal with that particular issue! I assume once I get them set correctly they won't have to be touched again. Unfortunately, I don't think there's a 'factory setting' number of turns to be found anywhere (since manuals ignore them altogether) so they'll have to be dialed in manually.

  9. #9
    Cyg Guest


    call up the mikuni dealer in janesville they have the micro fiches there and he will tell you the right number of turns too turn them out.
    I bet he would.

    thats him there

  10. #10
    Smellii Guest


    Hey, t3rmin (if you are still around). I'm having the same issue with my mixture screw threads. Did either of those taps work? Which one was the correct size? Thanks!

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