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vibration?

Discussion in 'XJ Technical Chat' started by livingdeadlyxj650, Apr 19, 2013.

  1. livingdeadlyxj650

    livingdeadlyxj650 Member

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    Dang it feels good to be on a properly tuned xj. Runs like omg fantastic. Rides pretty nice as well. And I thank you to every one here that has helped me to get to this point. Anyways! This is what's going on. Right at 60 mph. Im getting a vibration through the seat of my pants and through my grips. Feels like a gigantic cell phone vibrating. But goes away at 58 or 61. Stange stuff. Any ideas guys?
     
  2. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Ride at 65. Inline fours buzz. Perfectly normal behavior for your engine
     
  3. mtnbikecrazy55

    mtnbikecrazy55 Active Member

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    Yup, xj's harmonic range per say.

    It's usually around 4500 -4700 rpm, not speed dependent.

    Now that you have it properly tuned up, kick it down to 4th gear and hear the sweet sweet sound of a properly tuned xj at 6k :D
     
  4. livingdeadlyxj650

    livingdeadlyxj650 Member

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    Oh wow? So everyone experiences this? And its not detrimental to anything? Im having a hard time letting the bike wind out. Makes me cringe. Im used to v8 rpms lol
     
  5. waldo

    waldo Member

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    Think Ferrari not Chevrolet
     
  6. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    Channel your inner Ralf Schumacher.

    ALL XJ's have a "harmonic" RPM range, as pointed out above.

    That being said, the effect can be greatly minimized by making sure the bike is truly in tune: valves in spec, float levels accurate, vacuum sync-ed to perfection, etc.

    The more out-of-tune the bike, the wider the RPM range and more noticeable the "buzz range."
     
  7. livingdeadlyxj650

    livingdeadlyxj650 Member

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    Check check check and am doing tom the vac sync. Its the only thing I lack. But it runs supreme as is. Lol my buddy is lending me his gauges tom. I just rode it and yeah it is in the rpm range.cool beans. Thanks guys.
     
  8. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    One more thing of note. Make sure to check the bolts and bushings (on rubber-mounted engines) for your engine mounts. Excessive looseness will alter the "buzziness" that you feel, usually not for the better.
     
  9. Polock

    Polock Well-Known Member

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    drill holes in the crankshaft :)
     
  10. livingdeadlyxj650

    livingdeadlyxj650 Member

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    LOL that member helped me do that earlier that week. Drilled out the end of every lobe on the cam aswell ;) ill check the mounts not a bad idea.
     
  11. RonWagner

    RonWagner Member

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    A harmonic is defined as when the natural response equals the forced response; it is what makes a bell ring, a radio work, and bridges collapse. Engineers try to design engines so the harmonics are not in the normal operating range; unfortunately the XJ and a lot of bikes have such a broad rpm range the harmonic is going to be somewhere. The range they chose is a bit annoying but they chose it for a good reason. The faster the engine runs the greater the stretching and compression forces on the rods and these forces are greatly amplified at resonance (the first harmonic) and can potentially worse at the second or third. Harmonics have multiples so there is goung to be a second one at double that rpm which is just outside of redline. Therefore they chose the lowest rpm range possible while still having only one harmonic. Conversly if they picked 4000 rpms for their first harmonic it would have been more mild but the second one at 8000 would be much more destructive.


    Bottom line: They designed the engine and frame to withstand the stress of the harmonic but it is best not to cruise in it; there is simply more wear going on. Just pass through it enjoy the feeling at 8-9K.
     
  12. RickCoMatic

    RickCoMatic Well-Known Member

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    Put some Rubber Wedges between the Cooling Vanes.

    Our Bikes came with them.
    Washing dislodges them, ... and you don't even know.

    [​IMG]
    Copyright YAMAHA Motors ©

    I'm pretty sure they are available from Len.
    Click on XJ4Ever.
     
  13. Mike82mxm

    Mike82mxm Member

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    that makes me laugh!!!!! sooo funny just outta nowhere
     
  14. Mike82mxm

    Mike82mxm Member

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    I had a 83 xj 650 bad vibes @ 100km/h but @ 101km/h they were gone.I was always told don't worry about it.so i never did
     
  15. waldo

    waldo Member

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    I have seen pictures of fellows who have drilled holes in their crankshaft and as far as I am concerned my crankshaft is fine without any additional holes in it. The one whole put in it by ( fill in the appropriate divine spirit that your religion says done it) is fine with me. Praise all that is Holy to you and yours. Amen
     
  16. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    That particular vibration is audible, and occurs from idle all the way to redline (on my bike anyway). It's caused by the length and placement of the cooling fins. Vacuum hose works just as well as the wedges to silence the ringing.
     
  17. livingdeadlyxj650

    livingdeadlyxj650 Member

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    Yeah mine Defo still has most of the blocks. And its Defo just a physical vibration. Lol
     
  18. RickCoMatic

    RickCoMatic Well-Known Member

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    I'd bet the Crank was "Balanced" during the Build.
     
  19. MercuryMan

    MercuryMan Active Member

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    You can also fill your bars to minimize the buzz or use BarSnake, and put rubber washers where your pegs meet frame. Proper tuning, good oil, and E'thing tight to specs will also minimize. If someone would make some nice polyurethane bushings for our bikes that would be ideal. Last resort ride at higher rpms!

    .
     
  20. RickCoMatic

    RickCoMatic Well-Known Member

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