r100gs -91 vibration at 3000-3500 rpm

Discussion in 'Airheads' started by KaaJa, Jun 28, 2016.

  1. KaaJa

    KaaJa Been here awhile

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    I have very noticeable vibration at 3000-3500 rpm on my gs. It seems to come from engine as it's vibrating at standstill when I rev it to 3000 and on the move when I pull my clutch and keep the revs at 3000. It also starts on engine breaking at 3500 and is at baddest at 3000.
    I've changed wössner 9.5:1 pistons, conrods are balanced and r100r exhaust, its stock otherwise.
    Just checked valve clearance and timing. Timing has something amiss, when I set it at 3500 at Z, then at idle its about middle between OT and S.
    Vibration is feeled strongly through seat and pegs.
    What should I check next? Do I need new beancan?
    #1
  2. 190e

    190e Long timer

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    Sounds like the advance mechanism springs are a little weak but you don't need a whole new bean can. Just replace the springs.

    I doubt in any case that an ignition timing error is the source of strong vibration. How is the carb sync ?
    #2
  3. Rob Farmer

    Rob Farmer Long timer

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    Does it rev out or stop revving under load at around 4k? Worth checking the diaphragms
    #3
  4. KaaJa

    KaaJa Been here awhile

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    Carb sync has been checked few thousand km ago and I'm going to check it again tomorrow.
    It revs to redline under load and pulls as it should, I think.
    I'm thinking on switching to Silent Hektik crank ignition, any thoughts on that? Or should I just replace springs on advance mechanism and be happy with that...
    #4
  5. 190e

    190e Long timer

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    Crank ignition is a nice thing to have but at a price. And you might spend the money and still have the vibration. If you have it timed right at full advance it's hard to see how the timing is the cause of vibration.

    Bad vibration in a particular rev range is usually a resonant effect so it may be worth checking the engine mount torque settings.
    #5
  6. GJUNKY

    GJUNKY Been here awhile

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    I had a real high buzz vibration at 3k - 4k on my R100GSPD that sync would not adjust out. It turned out to be unequal fuel level in the bowls. I had set the floats to shut off parallel to the casting. When I measured the fuel level it was way off.
    I reset the floats based on measuring the fuel in the bowl and the vibration wen away
    #6
    adrenal, Uke and Disston like this.
  7. Disston

    Disston ShadeTreeExpert Supporter

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    Doubt it's ignition related. Setting to the Z at high rpm is correct. New springs may help. For the most part they all do that. Probably the best improvement can be had in the carb balance. I like the one about balancing the floats.
    #7
  8. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    adjust valves (but In my experience tight valves more likely to effect idle), clean plug since they will be out for valve adj, set idle timing "S" mark and 2,500-3000 at "Z", using a strobe light should point out anything unusual with the ignition, sync carbs at idle and higher RPMs..

    after doing that then it gets more involved.......
    #8
  9. 190e

    190e Long timer

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    I've had the same experience due to floats of unequal weight. Setting floats parallel is a flawed method unless you already know that both floats weigh the same.
    #9
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  10. KaaJa

    KaaJa Been here awhile

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    -Float levels checked and adjusted, 28mm of fuel in bowl. there was about 0.5mm difference to begin.
    -Timing checked again
    -Carbs sync adjusted, was little off.
    -New caskets between exhaust front pipe and collector, old ones did not tighten enough and were loose.
    -Motor mounting bolts torqued to spec

    -vibe is still there, its worst at engine braking between 3500 and 3000 rpm. Could this be normal? Cruising at 4000-5000 it's absolutely fine. I have to test my buddys stock r100gs and compare.
    #10
  11. Big Bamboo

    Big Bamboo Aircooled & Sunbaked

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    I'd say it's normal to feel some vibration at that low of an rpm. Try keeping the revs closer to 4500-5000 for better engine braking and acceleration, too. Remember; there is an additional 10 HP to be gained between 5-6000rpm...
    #11
  12. Beemerguru

    Beemerguru Beemerguru...G/S guy

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    You don't happen to have anything knocking in the engine or is it just a vibration? Did this just start or gradually creep up on you? Worse case if do a top end job and replace the rod bearings and check everything part by part. I always balance the pistons rods, and wrist pins on my builds and never have a vibration up or down the range so you have something unusual..maybe
    #12
  13. KaaJa

    KaaJa Been here awhile

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    I don't think its knocking, just vibes. Rod bearings are new. Heads were rebuilt also, new valves and guides. I don't remember it being like this at start, I've driven it about 4000km after the rebuild.
    #13
  14. Peashooter

    Peashooter Older Dog

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    My GS was rougher than I thought it should be, and not as smooth as my old one was. Ended up finding the pistons were 2 grams different in weight while doing the top end, and had them balanced. But the biggest improvement was having the entire flywheel, clutch assembly balanced. Now it is pretty damn smooth. Also check your driveshaft. It should feel very smooth when you turn the rear wheel.
    #14
  15. Peashooter

    Peashooter Older Dog

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    Do you remember what you set the fuel level at?
    #15
  16. GJUNKY

    GJUNKY Been here awhile

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    Another thing to do is a compression test and leak down test to make sure everything is good with the rings and valves.
    #16
  17. KaaJa

    KaaJa Been here awhile

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    Compression and leakdown tests are next thing to do. I'm beginning to think that there is compression difference between cylinders.
    #17
  18. KaaJa

    KaaJa Been here awhile

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    Picture of sparkplugs, right one is looking bad?
    [​IMG]
    #18
  19. 190e

    190e Long timer

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    Neither plug looks great.

    I think you need to use new plugs to judge the mixture after the carb adjustments.
    #19
  20. KaaJa

    KaaJa Been here awhile

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    compression test results:
    Right:
    [​IMG]
    Left:
    [​IMG]

    Didn't get equipment for leakdown test yet.
    #20