990 engine vibration.

Discussion in 'Dakar champion (950/990)' started by Parx400, Jun 8, 2016.

  1. Parx400

    Parx400 Long timer Supporter

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    After reading some other threads I have a bad feeling its compression related. The bike has always used oil and when I got it i found evidence there was some sand in the air box. It had a shitty BMC filter in it when I got the bike that was not sealing well at all. The bike ran fine for over 10K miles after I got it. the last 3K the vibration has started.

    Compression gauge will be here after work.
    #21
  2. RoundOz

    RoundOz Plenty of seasoning

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    Easy enough to hook up a good pair of vacuum gauges. If you have the desmog done there are even two spare ports. That will give you some of the story. IR temp scan of each cylinder/exhaust might give some more clues. I have been chasing a vibration that may be chain or tune related. Have changed both recently and hope to have it back on the road some time in this decade.
    #22
  3. motoman250f

    motoman250f Been here awhile

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    I just asked a ktm shop about tracking air fuel through the bungs on my SE and they say they cant help with that. (got a vibration too) they suggested go to a dyno tuner and get someone with a sniffer and compare both sides. Yeah could be compression related but if its bad it will have a hard time starting and idling usually. compression would explain why its better when cold too.
    #23
  4. Parx400

    Parx400 Long timer Supporter

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    Starts fine. Idles a little funny. Never dies.

    Why would it run better cold with low compression?
    #24
  5. motoman250f

    motoman250f Been here awhile

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    compression goes up because air is more dense at lower temps.
    #25
  6. Hoologan

    Hoologan Been here awhile Supporter

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    Simply not true.
    #26
  7. Parx400

    Parx400 Long timer Supporter

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    Let's hear both sides of this.
    #27
  8. 4bikes

    4bikes Been here awhile

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  9. Hoologan

    Hoologan Been here awhile Supporter

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    Yes, there is a relationship between pressure and temperature (obviously), but that's not what I'm disputing.

    Charles' Law has no bearing on compression ratio. If a ring is blowing by or a valve is leaking, it's going to blow by or leak regardless of air temperature. Air doesn't suddenly become mayonnaise when it cools off 30 degrees and seal the leak.
    #29
  10. Parx400

    Parx400 Long timer Supporter

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    My non engineer logic tell me the blow by on rings would be worse when cold. Should't the rings expand as they get hot?
    #30
  11. Hoologan

    Hoologan Been here awhile Supporter

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    As long as the engine is running, it's hot af in there. We're not talking about the difference between initial startup and warmed up here.
    #31
  12. motoman250f

    motoman250f Been here awhile

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    when rings get warm the end gap shrinks and there should be less blow by through the end gap. so what your saying makes sense but if rings aren't sealing they could be broke, unround, damaged piston, etc.... I think most of what your talking about is outside temp not necessarily cold engine right? Hoologan, you need to reread, i'll cut you some slack until you look at this with a fresh mind.
    #32
  13. Parx400

    Parx400 Long timer Supporter

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    Woohoo. 240 and 235 on. The compression check. Glad that's not the problem. I'm going to adjust the semi tight intakes and see where that gets me.
    #33
  14. motoman250f

    motoman250f Been here awhile

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    Parx, I think I found my problem when someone timed the cams last after valve adjustment they got one exhaust cam installed out of time. they used the dimples not the "x" on the rear. so make sure your cams are timed correctly. How tight are your valves? cant be too much pressure or they would leak and give bad compression reading.
    #34
  15. Parx400

    Parx400 Long timer Supporter

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    The rear intakes are right on the boarder line of spec. I'm going to adjust those. Have not checked the front yet.
    #35
  16. Parx400

    Parx400 Long timer Supporter

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    All the valves were inspec except for the rear intakes. Installed new plugs just to pull that out of the equation. Going to sync the TB's and keep my fingers crossed.

    After that I guess its time to look at fueling. I just changed the fuel filters 3 K miles ago. they had been plugged and I had to limp the bike home.

    Going to pull my hair out after that.
    #36
  17. RoundOz

    RoundOz Plenty of seasoning

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  18. MortimerSickle

    MortimerSickle Semi-Adventurer

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    #38
  19. motoman250f

    motoman250f Been here awhile

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    Well hell, after reading that mortimersickle I can see our frustrations are microscopic!
    #39
  20. blutor

    blutor Adventurer

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    Try to borrow a complete exhaust system from a non vibrating bike.

    Maybe it's pulsation from the exhaust gases that's being restricted by your exhaust system.

    Or pull your exhaust system off, put on ear protectors and start the bike. Feel any vibes then?
    #40