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Mikuni BS34 air intake jets/passages

Discussion in 'The Garage' started by mcrowell75, Dec 5, 2010.

  1. mcrowell75

    mcrowell75 XS400 Enthusiast

    Can anyone help to identify where each of these air passages go and what they do?

    I am particularly interested in #4. There is a brass jet in there that I couldn't remove. I have seen it called an "air" jet. I think I read somewhere that it cannot be removed because it is pressed or cast into the carb body. Is that true? What does it do? What "circuit" is it for?

    I'm not sure if I broke a forum rule by using houghmade's picture. It was very clear and better than anything I could have taken.

    Attached Files:

  2. drewpy

    drewpy Excess twin Top Contributor

    4 is the pilot air jet
    3 is the mains air jet
    1 2 5 are atmosphere vents 1 = vents underside the diaphram (i use it to test the integrety of diaphram and that the slide can move easily in the carb body.
    5 and 1 vents the carb float area!
  3. HoughMade

    HoughMade XS400 Guru Top Contributor

    And the whole things was in my garage. ;)
  4. cturek

    cturek XS400 Addict

    Here's a diagram of the internal passages for these style of Mikuni's.

    Attached Files:

  5. mcrowell75

    mcrowell75 XS400 Enthusiast

  6. HoughMade

    HoughMade XS400 Guru Top Contributor

    Feel free to use my picture- I was just kidding. I'm glad it was useful.
  7. drewpy

    drewpy Excess twin Top Contributor


  8. Drockcity

    Drockcity XS400 Member

    anyone know of any good sites that sell aftermarket carbs/ignition? looking to get better flow/spark and a little better performance?

  9. bcware

    bcware Well-read Top Contributor

    mikesxs.net has pretty much everything you need to jet your stock carbs or to upgrade to some aftermarket carbs.
  10. FUBAR

    FUBAR XS400 Addict

    haha drewpy..
  11. swascott1981

    swascott1981 XS400 Member

    OK so I have an 81 xs400 special my problem is the idling so I tried settingthe air mixture screw on the Carb 3 turns and adjusting the throttle cable til its at 1500 Tom's but it wont stay at 1500 it continues to go up and down to 4000 to 3000 and back down to 1500 any suggestions would be great
  12. HoughMade

    HoughMade XS400 Guru Top Contributor

    You don't adjust idle with the throttle cable. It's adjusted with the idle air screw which is between the carbs, on the back side just above the number "5" in the first pic. The throttle cable should have free play so when you let go, there is absolutely no tension on the cable.
  13. swascott1981

    swascott1981 XS400 Member

    OK well I know where the idle screw is I've tried adjusting it but it seems to it just not right maybe its vibrating loose but thanks for the tip
  14. HoughMade

    HoughMade XS400 Guru Top Contributor

    It is normal to have some climb in idle rpm as the bike warms. After i shut off the enrichener all the way (choke) and the bike is still cool having just run for a couple of minutes, it will idle around 1000rpm. After it warms up, it will vary from 1200-1400 rpm (give or take)- and there is nothing wrong with that.

    An idle up around 3000 rpm if everything is adjusted indicates a lean condition from adjustment, air leak or idle circuit passages being dirty. I would turn the idle mixture out to 3.5 turns, make sure there is free play on the throttle cable, good plugs properly gapped, just let it idle to warm, then use the idle air screw to adjust to 1200 rpm. If the idle still climbs too high, then check for air leaks. If there are none, the carbs need cleaning.

    BTW- the idle air screw should have a spring on it which will prevent it from loosening, If not, get one. Also, if it is vibrating loose, the idle will get lower.
  15. swascott1981

    swascott1981 XS400 Member

    Thanks hough I think I got the idle right now ya see I would set the idle screw then turn the throttle cable boy was I wrong but this is my first bike so I'm learning
  16. volcomskater77

    volcomskater77 XS400 Chopophile Top Contributor

    Hi guys,

    My name is Kevin and I am Volcomscaters dad. I am working on lighting up his XS400 bobber now, (he's out skiing), but have a problem. The carb bowls are not filling from the tank,(gravity, no vacuum). When I look at the float setting (1.26"), it appears the needles have a spring loaded pin into the needle itself. The spring pressure is sealing the needle to the seat at the spec'ed dimension!! Ideas??? Thanks. :banghead:
  17. drewpy

    drewpy Excess twin Top Contributor

    Hi Kevin,
    The spring wraps around the tang so it positivly pulls away from the seat and puts it back. Not all carbs had them (none on my '78) which I had to get the correct one as the spring version was too tall.

    hope that helps
  18. volcomskater77

    volcomskater77 XS400 Chopophile Top Contributor

    Hi Drewpy<

    Thanks for the confirmation. What kind of needles did your 78 have? I've never seen needles like these with a sping loaded pin IN the needle. All the carbs I've worked on have the little wispy spring that drags the needle in and out of the seat. With the fact the bike wasn't runniig when it was purchased, who the heck know what it should have. Are the standard needles readily available? Thanks.

    Volcom Sr!

  19. xschris

    xschris XS400 Guru Top Contributor

    The one on the left is a 77-79 needle and the one one the right is a 80-82
    Last edited: Sep 4, 2012
  20. volcomskater77

    volcomskater77 XS400 Chopophile Top Contributor

    dad, i got new needles with the jet kit. so they should be the stock size. i hope... and thanks chris, mine are the ones on the left. i know that the Hayes manual is wrong about the float height i believe. just dont remember what the dimensions were.

    and your welcome for letting you use my online user identity dad. asking wouldnt have really hurt.:boxing: come at me bro! :laugh:


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