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5k rpm vibration

Discussion in 'XJ Technical Chat' started by Rob A., Aug 3, 2018.

  1. k-moe

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

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    The Seca 750 bars have a steel plug welded into the right end of them. It's heavy enough to act as a bar end weight, so you'll only need one weight for the left side.
    I filled my (aftermarket) bars with butylene rubber caulk to dampen the vibration, which is something you can do to the stock bars. Use a lenght of vinyl tubing so you can start the fill on one end and retract the tubing as you go to make sure that all of the bar is filled.
     
  2. Rob A.

    Rob A. New Member

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    Well I removed the exhaust and reinstalled it making sure everything was tight and secure I even added new bushing to the rear exhaust mounts. I also added new bushings to the fins of the engine. It seemed to help but the buzz is still there about 5k. I guess my next option is too check the valves??? Any thought?
     
  3. k-moe

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

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    Did you do this already?
     
  4. Rob A.

    Rob A. New Member

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    I was wondering about carbs I noticed it running a little different. I haven't synced them yet because it was running so good and the bike would come unleashed at 6k.
    It seems strange that I never had vibration or a buzz before I broke an exhaust bolt? One of the nuts came loose and the other one must have broke due to pressure and stress.
     
  5. k-moe

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

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    The carbs can come out of synchronization over time. It's worth having a look.
    Even a bit out of synch enhances the buzz, but won't affect performance enough to notice.
     
  6. Rob A.

    Rob A. New Member

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    Ill have to try that I've never synced carbs before ill have to locate some gauges and try it any advice on syncing them? It doesn't sound too hard to do.
     
  7. Tim O

    Tim O Active Member

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    Duuuuuude.... Do It!
    Big difference.

    Believe there is a good rundown in the DIY area...
     
  8. Rob A.

    Rob A. New Member

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    Thanks I appreciate the info. Ill give it a try.
     
  9. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    There is an inherent buzz/vibration/resonance around that rpm....

    Sometimes it easiest to stay just above or just below it.

    Also, though, the first thing I would check---would be the clamp that holds the right-side chrome heat shield/trim onto the collector box by your right foot. They are notorious for having the front ring break, the the shield itself will buzz like crazy around that rpm. Easiest way to tell.... When you're at the rpm and your buzzing away, just turn your toe in against it and see if it stops
     
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  10. Rob A.

    Rob A. New Member

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    Ok thanks I will
     
  11. maybe4

    maybe4 Member

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    same exact thing on my 82' XJ650. they are just a bit buzzy at that range - and I HAVE tried to fix it. Now I just do what other people do and just avoid that RPM range although it is annoying to have to do that - especially since we are missing that 6th gear. Same RPM range (except mine is 4,800-4,900 RPM's then again at 5,200 - 5,400 RPM's. I've tried a lot of things - lead shot in the handlebars, vibranators, etc. what helped the most was dual density grips which are made to absorb vibration (Pro Grip 724's is what I bought) and then I put Grip Puppies (foam grip over them). Very nice feel, I like the wider diameter of the grips with the Grip Puppies on there. I think the only thing that could stop it since it's coming from the engine (harmonic distortion) would be a counter-balancer. I've been meaning to replace the rubber engine mounts since they are old and hard but I still doubt it would do much. inline fours have an "inherent second order vibration that can't be eliminated no matter how well the engine is tuned"......unless you have a counter balanced shafts
     
  12. maybe4

    maybe4 Member

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    wait.....you never felt any vibration until you broke an exhaust bolt ? I have three of these bikes, and hogfiddles has probably had hundreds, they do vibrate ever so slightly in that RPM range. and when I say vibration, it's not something you can see, you may feel it a little in footpegs and after a while in that range my hands will be a little numb. my bike is synched and tuned and runs great, I don't think any amount of tuning will get rid of the harmonic distortion created by a running engine
     
  13. jayrodoh

    jayrodoh YimYam Premium Member

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    I have/had/will have again a 82 Maxim 650 as well. I rebuilt almost everything on the bike excluding the bottom end of motor (look at link in sig). Had the exact same vibration around 5k and tried almost everything to track down. Felt it most in the seat and the gauges would rattle but otherwise nothing felt out of place. The only thing I did not do was replace the rubber motor mounts. Could be the issue but never got around to it before the bike was wrecked. I just lerarned to avoid the range as I rode.
     
  14. k-moe

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

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    Fellas....that vibration was there when the bikes left the factory.
     
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  15. Rob A.

    Rob A. New Member

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    I just pulled the bike out of the shop about 2m ago where it sat for about 13 years ago when my dad bought it and there sat for 13yrs. After I got it going I ran it over 6k a few times and no buzz at all. Then I didn't run it too hard until recently where I would rev it over 6k here and there. Then one day I was coming up the hill and bang I heard the exhaust nut and stud bounce off the front fender then of course I had an exhaust leak. When I got home (which was only a 1/2m away) I checked and the other existing nut was backed off. Once I fixed everything and took it for a test drive the buzz was there and still is.
     
  16. Rob A.

    Rob A. New Member

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    It's possible I just never felt it before but Im sure I would have questioned the buzz.
     
  17. maybe4

    maybe4 Member

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    I don't know, I could easily see not noticing it running around town on it - my brother and I switched bikes for a couple of hours while riding this summer and he couldn't feel it at all in the handle bars - exact words "I don't know what you're talking about" - then I told him to go gradually get up to 4800 RPM's and stay there and pay a attention to the foot pegs as you come into that range, then he conceded that he could feel it. I agree with K-moe, the buzz was there when they left the factory. I wouldn't be trying to solve it at all but on long rides the hand numbing is annoying to me.

    I have an extra set of handlebars so I think I'm going to try Bar Buzz Kill and also heavy bar end weights. NOT to stop the vibration mind you (because that's coming from the engine) but to absorb/counter it
     
  18. Polock

    Polock Well-Known Member

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    problem solved
    [​IMG]
     
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  19. turpentyne

    turpentyne Active Member

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    As we say in programming, “Not a bug— it’s a feature!!”
     
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  20. Tim O

    Tim O Active Member

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    I just met up with a guy today that has an 83 Seca 750... he's not on this list at all. When we were comparing notes on our bikes I think it was the third thing he mentioned as "going on" with his... described everything here perfectly.

    The exhaust thing listed above makes me wonder... we've been talking about engine mounts and everything else, maybe it's the pipes coming off the front and then to the collector that only has one bolt holding the whole thing down there.

    My bike has the vibe but I also replaced every gasket in my exhaust from front to back. I also never-seized every bolt and gasket. Maybe once everything is loose, the collector / pipes are free to vibrate... with a decent size moment arm to that one bolt and then you are stuck with it. That would agree with the "It wasn't there before disturbing the pipe(s) and gaskets," etc. There was enough corrosion at every bolt / joint to keep everything solid and once disturbed that was it.

    I might even suspect that the harmonics was the reason for the stud parting in the first place.
     
    Last edited: Aug 10, 2018

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