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Old Bike, New Topic

Discussion in 'XJ Technical Chat' started by Old_Ironsights, Aug 22, 2014.

  1. Old_Ironsights

    Old_Ironsights Member

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    ... at least, I couldn't find answers in a Search.

    So, I have this '81 650 Maxim that I use as a Commuter. I'm sure there are lots of little things that need rework - like intake boots (looking pretty skanky), a little oil weeping @ the Head Gasket, Carbs that need re-adjusting/syncing for 5,000 ft, etc.

    But what has me most puzzled is what I call the "5000 rpm Harmonic"

    In any gear, at precisely 5000 rpm, the tone/vibration of the engine changes dramatically. It goes from Smooth to sounding/feeling like it's going to grenade, then back to smooth all within about a 200 rpm range.

    Acceleration or deceleration, it doesn't matter. the Harmonic happens at 5000 no matter which side of the gauge I approach it from.

    This is particularly annoying because, for whatever reason, my XJ runs 60mph @ 5000rpm - which means I'm transitioning across this evil 5000 rpm Harmonic A LOT on the highway between the small towns around here.

    Has anyone ever heard of such a thing? Is a balancer out of whack? What's gong on? :?:
     
    Old_Ironsights, Aug 22, 2014
    #1
  2. TIMEtoRIDE

    TIMEtoRIDE Active Member

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    4-CYL engines would appear to be perfectly balanced, as equal masses are rising and falling, but they have a "rocking couple" due to connecting rod length and actual piston speed. The Buzz is at double crank speed.

    Enjoy the buzz.

    Some have said a running carb sync helps get rid of some of the buzz.
    You can also fill your handlebars with bird-shot. Or BarSnake.

    Welcome to the site !!
     
    TIMEtoRIDE, Aug 23, 2014
    #2
  3. Old_Ironsights

    Old_Ironsights Member

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    Well, as long as it not going to cause my engine to fly apart then I guess I'll live with it.
     
    Old_Ironsights, Aug 23, 2014
    #3
  4. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    It can be greatly minimized by getting the engine in a proper state of tune.
     
    bigfitz52, Aug 23, 2014
    #4
  5. PilotSmack

    PilotSmack Active Member

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    My engine used to rattle lots below 5000 rpm, but is pretty consistent since her rebuild. However, her exhaust sound is definitely different now, featuring this neat little howl above 6k. I like :)
     
    PilotSmack, Aug 23, 2014
    #5
  6. Stumplifter

    Stumplifter Well-Known Member

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    Stumplifter, Aug 24, 2014
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  7. TIMEtoRIDE

    TIMEtoRIDE Active Member

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    I got my terminology wrong, quoting "Tony Foale" -

    ". . . Although the primary forces balance out due to the opposed
    motion of the pistons, the secondary forces all add together. No rocking couples are produced."

    The "buzz" is the secondary forces - - the sum of a remainder of an unbalanced equation inherent to the XJ.
     
    TIMEtoRIDE, Aug 24, 2014
    #7

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