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1975 xs650b fuel/air mix screw setting?

Discussion in 'The Garage' started by Buckminster, Jun 22, 2013.

  1. Buckminster

    Buckminster XS650 Member

    My 1975 xs650b is getting real close to running well. I just worked my way through most of a major tune-up and it still is running a bit rougher than I'd like. I'm wondering about the fuel/air mix screw setting on the original B38 carbs. The original manual calls for "1 3/4 turn" while all the other specs for this bike (that I have found on line) call for a 3/4 turn. Maybe the manual is saying (1) 3/4" turn? Anyhew, It does seem to run better with 1 3/4 turns and begins to backfire and bog-down at 3/4 turns, but that seems like a big difference and I'd rather not burn something up running it lean. Does anybody have any knowledge on the issue?
     
  2. retiredgentleman

    retiredgentleman XS650 Guru Top Contributor

    Here is a link to read from the Tech section:
    http://www.xs650.com/forum/showthread.php?t=43

    There is a table (from 5twins) that shows what the specs are for each year. The carbs changed design every 2 years or so. The setting for the mixture screw is just a starting point, and each engine may be different. These stock sttings are for stock air boxes/filters and stock exhausts. If you have modified intakes or exhaust then, you would need different jets and different mixture screw settings.
     
  3. 5twins

    5twins XS650 Guru Top Contributor

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    Yes, set the screw where the bike runs best. Don't just put it at one spec or another and call it good. As RG said, those are just recommended starting points or initial settings so the bike will start and run. From that initial setting, you need to fine tune it and find what's best. You say it runs better at 1 3/4 but that may still not be ideal. You need to try 1 5/8, 1 7/8, 1 1/2, etc. Experiment and see what works best.
     
  4. XSLeo

    XSLeo XS650 Guru Top Contributor

    If your 75 still has the stock ignition and carbs using the dead cylinder method you can set the mix screws right.
    With the stock ignition just unplug one plug wire. adjust the idle speed on the other cylinder as low as you can and still keep it running.
    Now turn the mix screw in slowly till it starts to stumble or miss, whatever change you find in how it idles. Now turn it out till the idle stabilizes. Turn 1/8 turn more. Now rehook the plug wire and unplug the other. Repeat the procedure on the other carb.
    Once you have both mix screws set, with one plug wire unplugged, engine idling back out the idle speed screw very slowly till the engine quits. Swap sides, repeat the idle speed till it quits. This syncs the carbs at idle. Now with both plug wires hooked start the engine, the idle will be high. Now turning both idle speed screws the same amount. I like 1/8 turn at a time to set the idle speed to 1200-1500 rpms. This maintains the sync.
    Now with all that done give the throttle a quick twist, Does the engine come up off idle smoothly? No stumble or hesitation. If it does then your good to go. If it does stumble or hesitate turn both mix screws out 1/8 turn. Another quick twist. Better? If not another 1/8 turn on the mix screws.
    If it takes much more than that you might need to go one up over what you have for a pilot jet.
    If you change pilots you need to reset the mix screws as above then test.
    Leo
     
  5. Buckminster

    Buckminster XS650 Member

    Thanks guys! I can't thank you enough for the feedback. I am really excited to get this thing dialed in and will keep you posted.
     
  6. Buckminster

    Buckminster XS650 Member

    XSLeo, I think I may be doing something wrong on your dead cylinder method and would greatly appreciate some more input. I do have stock carbs/jets and ignition. Probably getting a bit ahead of myself, when I start with the factory 3/4 mix screw setting and pull the opposite plug, the left side kills at about 1000 and the right side idle screw backs all the way out and maintains an idle of about 1000 (on this side I need to put pressure on the throttle arm to get it to kill just below 1000.) With both cylinders going it idles OK at about 1400 and pulls to 4000 OK but probably not as great as it should. So, when I went back to adjust the mix screws per your post it seems I can turn the mix screw "in" until it seats on each sides and not cause the bike to stumble or miss. Only turning out causes the bike to stumble. I assume that turning in causes the bike to run more lean. Consequently, I abandoned the carb tune for now hoping I could track down some more info about my next move. As a side note, the carbs were recently cleaned. I also checked for air leaks with some carb cleaner and everything looked good. Any thoughts?
     
  7. 5twins

    5twins XS650 Guru Top Contributor

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    Your mix screws aren't behaving correctly. Closing them should about kill the motor. Check your float levels. If they're off, the mix screws can have little to no effect (like yours are doing).

    You may not have enough freeplay in the throttle cable on that side that won't go below 1000 RPMs. That could be why the speed won't drop any more unless you physically push down on the cable arm.

    Once you have the mix screws set and the idle speeds matched, you'll need to sync the carbs for higher speed operation. To do that, you'll want to match the amount of freeplay between the cables. Set them so they have about 1/8" to 3/16" of play, and the exact same amount on each one.
     
  8. Buckminster

    Buckminster XS650 Member

    Thanks again. Despite this hiccup, the bike is still running the best it ever has for me. Before I rip into it again I wanted to pick your brain a bit more. When I was previously adjusting things, I loosened the throttle cables - so, I don’t think that was contributing to my problem. I will definitely recheck my float levels, but I just set them a week ago with a micrometer. I also just replaced the mix set screws with new ones from a Mike’s kit. The new ones were a visual match to the originals and if my memory serves me correctly, I think the bike behaved the same with the old mix screws. Are there other likely culprits I can check once the carbs are pulled? Is it possible that there could be an issue with the butterfly assembly?
     
    Last edited: Jun 25, 2013
  9. 5twins

    5twins XS650 Guru Top Contributor

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    You mentioned rebuild kits from Mike's. Did you install the Y-22 generic needles included in the kits? If so, you'll want to take them out and put the originals back in. The Y-22s are not suited for your '75 carb set. They're about 5mm too short which will make them way too rich.
     
  10. Buckminster

    Buckminster XS650 Member

    10-4 5twins. I cleaned the old ones and use the new one as fancy tooth pics for guests.
     
    Last edited: Jun 25, 2013
  11. weaselbeak

    weaselbeak XS650 Junkie

    I never use a micrometer because all floats and needles do not act the same. I pull the bottom plug and insert a threaded tube made from a 6mm bolt. Put a clear fuel line on and run it snugly up the side of the carb. Do this on both carbs. Then drop fuel into the infeed. The fuel will fill the lines exactly as high as the fuel in each bowl. Adjust your floats to get the SAME level on both lines. The level should be right where the float bowl gasket sits on the carb body. A hair high is better than a hair low.
     
  12. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Hello Buckminster, this may help with the fuel level "clear tube method":

    http://www.xs650.com/forum/showthread.php?t=27182

    Since our bikes and carbs are so old, you may have worn throttle shaft bore holes, preventing the butterflys from closing properly. Would suggest recentering and reseating the butterflys. Couple good threads on here in this, try forum search "butterfly".
     
  13. Buckminster

    Buckminster XS650 Member

    Alright, I just spent a couple hours pulling the carbs to take another look and am only now seeing the last couple posts on the clear tube method so I have not yet run the test. Let me restate where I'm at: I have original/oem parts throughout (127.5 main, 4n8 needle, 45 pilot and am using the factory 3/4 mix screw setting for reasons mentioned below). I reset the float levels a bit from where I previously had them, but only a few mm. This seemed to help the bike idle maybe another 100 rpm lower than before. When I pull the wire from the opposite plug I can get each side down to 900rpms before it kills. I then turned the idle screws back up in tandem to 1400 and it sounds pretty good to me. When I revved it up to 4000 there was slight backfiring on the return so I turned in my mix screws a 1/4 turn in on both sides and it seemed to smooth it out. I am tempted to call it good enough for government work and move on to other non carb items. However, as previously stated, turning in my mix screws all the way till they seat doesn't kill the bike and only seems to speed the idle. Also, I can still back the idle screw all the way out on the left side without killing the bike, almost but not quite. How severe are these symptoms and I'm I in danger of burning something up because its running leaner than I think? Had I not come across this forum with all you guys I probably wouldn't have even given it a second thought. This is my first 650 and its just tough when you've never heard one that really dialed in to compare to.
     
  14. 5twins

    5twins XS650 Guru Top Contributor

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    When you back the idle speed screw out all the way so it isn't touching, the butterfly should close completely and that cylinder should quit. Sounds like your butterfly is not closing all the way. It could be off center and binding in the bore. The more they close, the more they contact the sides of the bore, and the more prone they are to bind if not centered properly.

    When you turn the mix screw in, that leans the idle mix. The popping and backfiring on decel is usually a lean indicator. It seems you've turned the screw the opposite of what's called for. Did you try it 1/4 or 1/2 turn more open?

    A few MM change in float setting can have a major effect. The spec for your '75 carbs is 24mm ± 1mm. I wouldn't set it outside that range, in fact I'd stay @ 24. In case you're not aware of it, when you set floats, you should measure twice, once from each side. Many times the two "bulbs" are twisted or tweaked on the mount and sit at different heights. If that's so and you only measure one side, your fuel level may not be what you think you're setting it to.
     
    AKOxTeam likes this.
  15. Buckminster

    Buckminster XS650 Member

    If my memory serves me correct, when I backed out from 3/4 to 1 turn the backfiring worsened on the decel. I have no idea why and will double check so that I am not confusing my directions. Having said that, moving in to 1/2 total turn out on each side sounded better. As for the butterfly, there was some slight black tarnish around the right one, but this is the side I can get to kill with the idle screw. The left side looked visually clean at the butterfly but this is the side that will not kill with the idle screw. How low is the bike supposed to idle on either side to tune stuff? I would guess its unlikey that the butterfly assembly got tweeked on both sides and they do both kill right around 900rpm. I can say that because at the side that I need to apply slight pressure to the throttle arm to kills the bike right at 900.

    Thanks for the advice on the floats. I'm still using a micometer and there's a small amount of guess work. Next time I pull the carbs I will use a steel rule or I might poke around in a hardware store to see if I can scrounge up the items for a clear tube test just to make sure. I did twist some uneveness out of one of them that I didn't catch before. Getting closer.
     
  16. katmol

    katmol XS650 Enthusiast

    Hey 5twins, I just bought a nice 1975 XS, how can I tell if the previous owner changed the needles or not?

     
  17. 5twins

    5twins XS650 Guru Top Contributor

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    They have a number stamped on the side. Original Mikuni needles will also have their trademark "square-in-square" emblem stamped on there.

    [​IMG]
     
    Last edited: May 21, 2020
    gggGary likes this.
  18. Christen

    Christen XS650 Member

    Entering the conversation from the side here. I am having some similar carb adjustment and/or ignition issues on my 1975 XS 650B. I was just about to ask about the Y22 needle length in the rebuild kits and I'm relieved to know that the original ones should go back in. Thanks
     
  19. Christen

    Christen XS650 Member

    If you have a combination square, you can set the end of the rule in the gasket depression and check the level of both floats against the bottom edge of the square.
     
  20. 5twins

    5twins XS650 Guru Top Contributor

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    Yes, I don't know why many of the later carb rebuild kits include that Y-22 needle. They've been like that for years. That needle is only suited for the early '70-'73 carb sets with the raised floor in the main bore. They take a shorter needle like that. Once they dropped the floor in '74, a longer needle was required. And then in '78 when they redesigned the slide with the needle mounting higher up in it, an even longer needle was needed .....

    [​IMG]
     

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