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Discussion in 'The Garage' started by Scotty4, Aug 15, 2020.

  1. Scotty4

    Scotty4 XS400 Member

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    Hi all, name's Mike and after years of reading on here, decided to join. I have my grandfather's 1978 XS400 with only 566 miles on it. He aquired it from a neighbor and only rode it a few times before parking it in the 80s. We were gifted it in 2004 and it sat while I was growing up.

    Now that I am home after being deployed, its time to get it running and enjoy it. Tires are new and the carbs were cleaned and synched by a friend. Looks like it needs a new battery and sparks and another carb cleaning and I should be on my way.
     

    Attached Files:

  2. xschris

    xschris a lifestyle not a trend Top Contributor

    Welcome to the forum. I hope you keep the bike all stock. These are very hard to find in this shape. I do see some one at some point changed the stock seat to what I think is a Saddlemen Double-Bucket Touring seat.
     
    Scotty4 likes this.
  3. Scotty4

    Scotty4 XS400 Member

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    There is a bit of surface rust on some of the chrome which I'll work on cleaning up. Was thinking I may need to change the rectifier eventually and might try to source some calipers for the front.

    I have a second 78 that is blown we were intending to use the electric start from but I like it stock. My grandfather passed this year, I want to keep it in his honor.
     
  4. Scotty4

    Scotty4 XS400 Member

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    Being new to this and carbs in general, do I need to drill out the idle air screw? The engine bogs and dies anything above idle. Idle is set at 1200 or so and it kicks to life real easy.

    Pictures of both sides of my carbs. If someone could point me to a thread or in the right direction, that wiuld be swell.
     

    Attached Files:

  5. amc49

    amc49 XS400 Addict

    If it idles then no need to mess with the drilling. All those screws do is set idle mixture. If idling fine and as soon as you open throttle more there are issues, problems are likely elsewhere. Bike sitting that long would have me suspecting the diaphragms on the slides first.
     
    Scotty4 likes this.
  6. Scotty4

    Scotty4 XS400 Member

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    @amc49 thanks. Just asked my dad about if he had it running and apparently it was doing the same when he first got it.

    Aside from them being synched, doesn't sound like much else has been done.
     
  7. Scotty4

    Scotty4 XS400 Member

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    Both diaphragms had this dry rot around the outside. Ordered some new ones yesterday. Heres a video of it idling and giving it any throttle.

     

    Attached Files:

  8. xschris

    xschris a lifestyle not a trend Top Contributor

    Make sure you got the ones for 77-79 carbs as the ones for 80-82 are larger.
     
    Scotty4 likes this.
  9. Scotty4

    Scotty4 XS400 Member

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    I ordered them from JBM. Got the M-73 which says it is for the xs400 with Mikuni BS34, slot measuring 73mm. Is this wrong? $47 isn't too tough on the wallet if I need to reorder. They were responsive thankfully.
     
  10. xschris

    xschris a lifestyle not a trend Top Contributor

    The xs400 used two different bs-34 carbs. The 77-79 used the mark 2 and the 80-82 used the mark 3. The 77-79 use a smaller slide than the 80-82. The outer part is the same 73mm but where the slide attaches it's not.
     
  11. Scotty4

    Scotty4 XS400 Member

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    @xschris the diaphragms fit perfectly. Everything is back together and operates exactly the same. If I hold the throttle slightly, the rpms come up a hair, any further and it dies.

    I supsect the carbs are not synched perfectly as it was never done while running. The left side is noticeably louder once idling. With the carbs out, to my untrained eye, it the butterflies looked slightly off.
     
  12. xschris

    xschris a lifestyle not a trend Top Contributor

    Make a manometer to sync them. I would make sure the carbs are clean and adjusted. Make sure the ignition points and advance unit are clean and moving as they should. Also do a compression test of the motor. You should get 150 psi or more. If it's under 125 your rings could be bad.
     
    Scotty4 likes this.
  13. amc49

    amc49 XS400 Addict

    With ear close to exhaust exit the two sides should sound close to the same intensity of noise and speak at the same rate as far as impulses. As in pum....pum....pum.... does not compute with pum..pum..pum..pum.. or pum..........pum.......... You want them both the same.The idle mixture screws should be very close to each other in the number of turns open or something wrong there.Then you adjust the individual exhaust rate using the cable adjuster on that carb.

    If you have good ears you can sync dead nuts using that alone on twins, I used to do it all the time, even got good on triples doing it.

    If not synced, when you slightly open the throttle one side will speed up the impulse rate but the other won't, it will sound messed up as the side not opening on time will still be speaking slow. You don't need any tool to pick up on that unless you can't hear.
     
    Last edited: Aug 28, 2020
  14. xschris

    xschris a lifestyle not a trend Top Contributor

    You can make a manometer for a few buck and it will save you a bunch of guess work. Mikuni put that sync screw between them for a reason. Use it. After that adjust you pilot mix screws for best idle. 3 turns out from a light seat is a good start. Be careful as the 77-79 carbs have a tendency to have the tips of them break off in the bodies. You may want to check that before anything just to be sure.
     
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  15. Scotty4

    Scotty4 XS400 Member

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    @xschris the screws on top of each carb next to the head? Mine have plastic caps and springs which only allow 180* of rotation.
     
  16. Scotty4

    Scotty4 XS400 Member

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    Thanks for the tips @amc49. I do have excellent hearing when it comes to machinery however, this being my first time, I'll use the manometer and get myself a baseline.
     
  17. xschris

    xschris a lifestyle not a trend Top Contributor

    You need to pull off those caps
     
  18. Scotty4

    Scotty4 XS400 Member

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    I see there is a normal screw below the cap, that for emissions? I'll pop them off tomorrow and make a manometer.
     
  19. xschris

    xschris a lifestyle not a trend Top Contributor

    Yes. Make sure your adjusting and testing with the air boxes on the bike. They will effect your fuel/air mixture greatly.
     
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  20. amc49

    amc49 XS400 Addict

    Be careful removing the idle limiter caps, on some designs the idle screws underneath are made to break off easily doing it as you are messing with emissions settings. Open the screw up some as removing the caps tends to put side force on the screw, you don't want that anywhere near the seat when it happens.

    I forget these use a positive linkage setup instead of individual cables, in that case any sync linkage or screw is the same thing as adjusting a cable. Usually one carb is chosen to be the 'base or master' and non-adjustable, the other(s) then set to match it.

    If you guys are talking a single position manometer then you are setting to pure idle only and only part of the work. You are only doing a half sync like that, complete has you barely opening the throttle to check the drop off matching of both (or all) cylinders as you just crack the throttle and you think both are even at idle but they both can then act different at pullaway. The difference if there can say more than one thing and you can't tell that without two gauges on a twin and even then they need to be a matched set. Why I never use gauges on a 4, too hard to get 4 of them at the same setting, far easier to use the tube type that has all cylinders pull off the same medium to be perfectly matched so there is no question as to the action of each cylinder. If you pay attention you can even pick up on say one cylinder being slightly weaker than the other to then match them a bit cockeyed at the dropoff to then idle smoother and give a better pullaway too. When engines get older the cylinders have different demands on the carbs at idle and off-idle and you can fix some of that to not be as irregular as it normally would be. Simply getting the curb idle vacuum the same will not do any of that.

    Of course, yours and do what you will.
     
    Last edited: Aug 28, 2020
    Scotty4 likes this.

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