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Ready to say goodbye to my 1980 400 special... unless

Discussion in 'The Garage' started by Tanners92, Jun 11, 2019.

  1. Tanners92

    Tanners92 XS400 New Member

    Okay guys I’m finally getting ready to give up on this bike unless any of y’all can solve this one. It’s a 1980 XS400 special. Sat in a garage for most of its life, only 6,000 miles on it.

    Im having a serious left cylinder constantly rich and right cylinder constantly lean problem. Which creates really poor running conditions. At this point I’m fairly confident I’ve gone through every single possible issue...

    Starting with:

    Carbs - I’ve tried THREE different carbs on the bike now. The original mikuni BS34 I totally cleaned, bought new and set ALL jets and screws to factor settings.

    • through out the course of these I played with various pilot screw adjustments, float height veriations, including butterfly adjustments.
    • I’ve pretty much concluded it’s not the carbs.. this has been a CONSTANT issue no matter how I adjust the carbs - even if I wanted to get right cylinder running rich I can’t.

    LEAKS -

    -I’ve replaced the carb boots on the head, and I’ve made sure the exhaust pipes have been fitted down tightly. I’m confident there is no vacuum leak.


    I have set the valves to factor positions. BOTH chambers get phenomenal compression. Upwards of 150psi.


    The bike is a SPECIAL 2 - which means it has an electronic ignition system, and is much more difficult to reset. I’ve set the timing the best I can to the point where it seems to run pretty consistently but I don’t think that would be a huge contributor to my problem.

    Coils and ignition system

    I’m getting good strong spark out both coils, I’ve tested the altornor, rectifier, all in working order. The system includes an “igniter unit” which I don’t know how to test, but I have no reason to believe that is the problem.

    I honestly don’t know what else to try. Any help will be greatly appreciated before I finally decide to just pass it on.


    Attached Files:

    Last edited: Jun 11, 2019
  2. xschris

    xschris a lifestyle not a trend Top Contributor

    Aftermarket mufflers wont help. Looks like your running xs360 carbs. What setting for those are you running? Still have stock air filter on the bike? Carbs synced with manometer? Make sure the pilot mix screws haven't broken off in the carb bodies. I run xs360 carbs on my 80 with stock intake and filters 77-79 xs400 head pipes and mac mufflers. My settings are 17.5 pilot jets, 130 main jets, slide needle on the third clip and mix screws set to 1.5 turns out, float height 24-26mm
    Last edited: Jun 12, 2019
  3. Tanners92

    Tanners92 XS400 New Member

    Hey xschris thanks for your reply

    You are correct, in that photo I was running with an xs360 carb (not sure what mikuni model it is)... and your settings seem right in ballpark of where I was working. I have since switched to the original carb that came stock on the bike.. and I’ve adjusted everything per factory settings. 42.5 pilot jets, pilot screws 3 1/2 turns out, 135 main jets, floats set at 32 mm (this is debated all over this forum, lots of people say 26mm, which flooded the carbs)
    ... regardless, both carbs, after doing my best to balance, AND borrowing and testing a carb off a GOOD running model, all gave me the same result: left/rich, right/lean.. so I’m quite confident it’s not the carbs..

    I AM running with original filters - I’ve checked for vacuum leaks - no luck..

    I haven’t yet tackled the possibility of after market exhaust causing the problem. Do you have any more information on why it may cause one cylinder to run rich and one lean?
  4. xschris

    xschris a lifestyle not a trend Top Contributor

    Float height for plastic ones are 22mm brass ones are 26mm. Always sync carbs with a manometer even using ones from other bikes. Every bike has different needs and vacuum per cylinder. Your TCI box might be bad causing out side to fire weak. Even tho you have spark with the plug out under compression it can fail or miss. If you have another xs400 with a known working TCI box I would swap it out for your and see what each bike does.
  5. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    Faulty float needles?

    Nice looking bike btw.

    My bike has points ignition so I cant speak of the TCI. Probably need an oscilloscope or something to test it.
  6. My bike was being flooded because regardless of float height the needle was not moving freely. Once I rotated the carbs it would hang and not plug the gas. Set correct float height and then make sure as you rotate carbs the needle still freely moves (when getting them back on bike it requires some finagling which was causing it to hang until I slightly bent the needle clips to adjust)

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