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1980 XS400 revving problems (videos)

Discussion in 'The Garage' started by K3ith, Sep 9, 2020.

  1. K3ith

    K3ith XS400 Enthusiast


    I got myself a 1980 XS400 and have problems with running.
    When I rev engine up quickly it wants to stall and when I do it slowly the revs will drop slowly. Better to understand from videos I put up to Google drive. https://drive.google.com/drive/folders/1IpVpMIz0JwpJea6CG__c5sZlx8_M6R6j?usp=sharing
    Don't look at the wiring mess, I'll start to rebuild the bike once I get it to run. :D
    I also had problems while running on idle. I rebuilt the carbs with rebuild kit and got it to idle almost fine, also bought new intake boots. But the reving problem is still there.
  2. BBS360

    BBS360 XS400 Guru Top Contributor

    Those pod filters are horrible and the exhaust isn't much better.
    Both will require carb jet changes. The pods look cheap. They may be blocking intake ports on the carb mouths.
    xschris likes this.
  3. amc49

    amc49 XS400 Addict

    If the wiring is something to ignition then it could easily be messing with idle.

    Strange how so many of you simply assume a 40 year old engine is in great condition then go after carbs, which while they may be a problem, may not be nearly all the issue when the basic engine has low compression to not want to rev or idle right to begin with.

    When you have an unknown quantity like old engine the first thing you do is to get a compression reading, it tells you what to go after next and whether you are wasting time and money on something that is never going to run right. Lost art I guess like so many other things now in today's I don't do that world where we wonder why nothing ever works right.

    CV type carbs literally demand an engine that is sealed up tight, the engine opens the carbs up by virtue of being in great shape which is what increases the draw the engine can make to open vacuum slides. When the slides slow down opening because compression is low then the carbs go rich and you have no idea of where your jetting is at all then.

    If the pods partly block off the positive pressure air port that opens the slides up much faster then the carbs will act all messed up as well.
    Last edited: Sep 10, 2020
  4. K3ith

    K3ith XS400 Enthusiast

    Yep, the exhaust is crap, i'll buy new one for next season, i live in Northern Europe, riding season is over soon. I found one guy from internet who is using same pod filters and he said that he changed main jet to 137.5 and it runs great for Him with these. Thats what I did too when rebuilding carbs.

    I'm not experienced on motorcycles and I'm not assuming anything, just carb clean and rebuild seemed the first right thing to do and so said many others. That's why I signed up to this forum, to get some advice and help. I'll try to put the original h-pipe before the filters and try to find compression tester for next time I go to garage.

    Thanks for help, I'll keep you updated when I get some new results.
    BBS360 likes this.
  5. K3ith

    K3ith XS400 Enthusiast

    Did compression test and 150-155 psi for both cylinders, which is good according to manual. Put original h-pipe between air pods. Idle is almost good and revs are dropping better then before. But still wants to stall when giving throttle faster.
  6. xschris

    xschris a lifestyle not a trend Top Contributor

    I would use black foam uni air filters if you must have pods. Much better than any "screen/pleated" type filter. Also as stated before those mufflers are very bad for tuning.
  7. amc49

    amc49 XS400 Addict

    If points and not electronic ignition then that can easily put you there too. Tune them up to the correct spec.

    (edit, finally listened to the vids)

    The popping/spitting back in last vid is fouled plugs, they are likely flooded wet or black. Clearly when the throttle is let off one cylinder only is running, the other is pretty much dead, the sync is miles off or carb on one side is messed up. The engine will never accelerate correctly like that.

    Do a complete tuneup and most of that will go away, you may need new plugs even if those were new.
    Last edited: Sep 14, 2020


    amc49 nailed it on this one. I listened to the video as well, and it's sounds to me like she's only firing off one cylinder. I think you should touch the exhaust and see if one of em isn't getting warm. It's could be something as simple as a bad wire from the coil. Check for spark, man.
    Last edited: Sep 17, 2020

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