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Hello All! Carb question

Discussion in 'The Garage' started by josephjhaney, Sep 12, 2019.

  1. josephjhaney

    josephjhaney XS650 New Member

    Hi all, my daughters boyfriend just scored a great deal on one of these classics. I had an xs 400 special when I was in college, so I was thrilled to see him get the 650, and she's in pretty darn good shape, of course the carbs were all gummed up, so I have some questions- First I've done my research and pulled them, cleaned the main jet, needle jet (removed from the body and cleaned) and the pilot jets. I'm trying to get her running properly now, here is where I am-

    It idles with the choke on, so that's good, and revs pretty well with a little hesitation with the choke on as well. Choke off, dies instantly. This is an 81, has the plastic floats. I can't for the life of me find the mixture adjustment screw on this thing, I've looked all over it, I just can't seem to find it, so that's where I'd like to start. Also I put pod filters on it since those stock airbox filters were a pita to deal with, but I did the tapered fins and it seems people are advising against that here, could that be part of my issue as well?

    Thanks in advance, looking forward to getting the kid on the road. (I own a Vmax, so my days of buying bikes ended with my first ride on her). :)

    Joe
     
  2. josephjhaney

    josephjhaney XS650 New Member

    Oh, and one other thing, where is there a rubber plug in the pilot jet hole in the carb? That seems counter intuitive to me, since that jet is obviously supposed to flow somehow?

    Joe
     
  3. 5twins

    5twins XS650 Guru Top Contributor

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    The pilot jet gets it's fuel supply from the side, pulling it from the main jet area. You need that rubber cap over it or the bike won't run right.

    Sounds like your carbs are still dirty, in particular the idle circuit. You can't do a proper idle circuit cleaning without removing the mix screw. Yes, there is a mix screw but it was capped over from the factory with a metal plug. You have to remove that 1st to get at the screw. It's located in the little tower on top, just in front of the chrome top cover .....

    [​IMG]

    The usual method for removing the plug is to drill a small hole into it (about 1/8") and thread a blunted sheet metal screw into it. Then yank the screw and plug out with vice grips. Take care to not drill too deep and go all the way through the plug. The mix screw is just below it and easily damaged if you do. The screw is just brass and drilling into it messes up it's screwdriver slot, making adjustment or removal difficult .....

    [​IMG]

    [​IMG]

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    As you can see in the 1st pic, there is a small hole in the metal plug. Drill a little bit and check your hole, looking at that little hole in the center. Drill a little more and check again, etc. When you notice that little hole in the center start to get larger, stop drilling, you're almost through the plug.
     
    gggGary likes this.
  4. josephjhaney

    josephjhaney XS650 New Member

    Great! Thanks so much for the quick reply, looks like I have more work to do tonight, but I'm glad to see this!
     
    gggGary likes this.
  5. 5twins

    5twins XS650 Guru Top Contributor

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    A couple more things I should probably mention - the mix screw is actually a small assembly. It consists of the screw, a spring, a small thin washer, and an o-ring on the bottom. You'll want to remove all these parts. The o-ring is often all mashed down into the bottom of the hole but you need it get it out. It probably needs to be replaced after being in there under compression for 30+ years. I'm sure it's all hardened up and flattened out. The size you'll need is 1mm x 3mm.

    [​IMG]

    For a stock or nearly stock bike that hasn't been re-jetted, these BS34s like their mix screws set about 3 to 3.5 turns out. After cleaning, I'd start with them set at about 3 turns out and fine tune from there if need be.

    The pilot jet delivers it's fuel/air mix into the main bore through 4 tiny holes. Three are grouped together by where the butterfly plate closes. These come directly from the pilot jet. The fourth is slightly ahead of the other three and is the mix screw outlet. A portion of the fuel/air mix from the pilot jet passes through or by the mix screw on it's way out this hole. You can adjust this flow with the screw.

    [​IMG]

    So, for the pilot circuit to function correctly, all four of these little holes must be clear. With the pilot jet removed, blow cleaner through it's hole with compressed air. Block off the mix screw hole with a finger so that the cleaner can only come out the four little holes in the bore. Alternately block off the little mix screw outlet hole and the 3 by-pass holes so cleaner can only come out the unblocked hole(s). You can also "back flush" the circuit by spraying cleaner in the mix screw hole and blocking off the 4 little holes in the bore. Cleaner will then get blown back out the pilot jet hole.
     
    arcticXS and gggGary like this.

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