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Need troubleshooting help - xs400 dies after warm-up

Discussion in 'The Garage' started by Carter Thomas, Nov 18, 2020.

  1. Carter Thomas

    Carter Thomas XS400 Member

    '82 xs400 Seca

    Hoping someone can suggest something I've missed. Love this bike and it really runs great, when it runs. Not a great pic of my bike but all I could find today

    Starts up on first bump of starter and runs strong...for a little while
    Stop for a light or make a low speed turn, it dies and won't start up again (usually happens on deceleration and usually backfires)
    Let it sit 30 mins - hour and it's all systems go again...for a little while
    Repeat above steps


    Replaced:
    battery (new)
    plugs (new)
    pick-up coils (new)
    plug wires (new)
    ignition coils (used)
    CDI (used)
    Also bypassed kill switch on bar
     

    Attached Files:

    Last edited: Nov 18, 2020
  2. Spitfire_X24

    Spitfire_X24 XS400 Enthusiast

    First of all, that is a good looking bike! We need more pics of that.

    When you say for a while, does that mean it runs good just until it's warm, or it runs for a half hour, an hour?
    And when it won't start are you still getting spark?

    Sorry for the additional questions, I don't have a lot of motorcycle experience, so the extra info helps. I'm sure one of the more experienced guys on here will be able to point you in the right direction.
     
  3. amc49

    amc49 XS400 Addict

    The key word is backfire. Yank plugs and if black they are fouled and something running rich. It fouls the plugs to not start back up without waiting.

    If so then run a compression test, an old engine will do that.
     
  4. Carter Thomas

    Carter Thomas XS400 Member

    Will run ok for anywhere from 10 minutes up to maybe 20. Don't think has never run more than 30. Doesn't always backfire but it does happen a lot. Sometimes will just shut off at idle. Mechanic has checked compression recently. I don't remember what readings were but he said it was ok. Bike has spent a lot of time at mechanic with extensive carb work. Pulled the plugs don't look too bad
     

    Attached Files:

  5. Carter Thomas

    Carter Thomas XS400 Member

    AMC - thanks for your help, but I'm not sure what you mean by this
     
    Last edited: Nov 19, 2020
  6. amc49

    amc49 XS400 Addict

    When the plugs foul from being black or wet the engine has to sit for a while for the excess fuel to dry up off plugs before it will start up again, it's the typical physics of a flooded engine. Too much gas not being burned.
     
    Carter Thomas likes this.
  7. Carter Thomas

    Carter Thomas XS400 Member

    thanks for the info, that's what I thought you were saying but wasn't sure
    I've never looked at the plugs right after it happens I'll pull them next time and see if they're wet
     
  8. Carter Thomas

    Carter Thomas XS400 Member

    Rode the bike again yesterday until it stalled out. Pulled the plugs immediately and both were clean and dry.

    What else could be going on here? Bike runs so good for short time and that's it
     
  9. amc49

    amc49 XS400 Addict

    Fuel filter stopped up can do it.
     
  10. Carter Thomas

    Carter Thomas XS400 Member

    hmmmm - filter is fairly new but I'll give that a shot
     
  11. amc49

    amc49 XS400 Addict

    EXACT compression numbers, Don't tell me the mech said it was good as it may mean nothing. Valves set lately? Miileage on motor?

    IF the engine is solid then a fuel or electrical issue. Consider where you get fuel as you have ethanol in it there. Water in fuel can easily do that.

    Check grounds and not just at battery. Frame too.
     
  12. xschris

    xschris a lifestyle not a trend Top Contributor

    Vapor locked fuel tank?
     
  13. Carter Thomas

    Carter Thomas XS400 Member

    That crossed my mind too. Tried running with fuel cap slightly open but no luck
     
  14. Spitfire_X24

    Spitfire_X24 XS400 Enthusiast

    I wonder if it is something heat related. I was having issues with my Motorhome generator, it would run fine for 20 minutes or so and then it would just stop charging, it would continue to run, but no power to the house. After much trouble shooting and checking of regulators and such, I won't go into all the details, it turns out that the generator as it warmed up would develop an internal short and kick itself off. Could there be something in your ignition system that would be causing it? Faulty coils maybe?
    Just spitballing...
     
  15. Carter Thomas

    Carter Thomas XS400 Member

    Definitely seems to me that it's heat related

    I've already replaced CDI, coils, plug wires and pick up coil though
     
  16. xschris

    xschris a lifestyle not a trend Top Contributor

    If your running pods and a non-stock exhaust the bike may be too lean. I would also check float height and you pilot mix screws settings.
     
  17. amc49

    amc49 XS400 Addict

    Have an extra new plug in pocket and quickly unplug a plug in head and plug the new plug in and grounded to head and check for spark with key on. Immediately when it dies of course.

    Same idea with fuel line to carb, have it easily removeable and quickly yank it at the the die to see if fuel runs out. Need to check carb bowls for fuel too.

    I've seen excess fuel line run too horizontal for a bit to not fill the bowls up and worse with low fuel in tank, fill it up. The bowls slowly get fuel and the engine use passes that up to run out of gas. Does that bike have the vacuum fuel demand valve between petcock and carbs? That will definitely do it if bad. Chokes leaking can do it too.

    Describe the die as accurately as possible..............like turning key off? You mention backfire. Does one cylinder run slightly longer than the other before dying? Big clue.
     
    Last edited: Nov 25, 2020
  18. Spitfire_X24

    Spitfire_X24 XS400 Enthusiast

    I agree with AMC49, gotta start to narrow down if it’s spark or fuel causing it.
     

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