1990 VFR 750 Engine Vibration

Discussion in 'Mechanics Garage' started by ride, Mar 31, 2006.

  1. ride

    ride New Member

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    Hey all. Just wondering if anyone has any insight into this little problem. I have a 1990 with 20,000 kms. Bike runs flawless, until it gets near and above 7000 rpms. I'm usually on the highway at this point, doing about 140-150 kms/hr in 5th or 6th gear. It feels like a bad wheel bearing, but if I pull in the clutch, all is smooth. Definitely a drivetrain thing. Also, don't know if its related, but the bike seems to get poor gas mileage. I also have a 1988 Yamaha FZ750, and when my brother I went out riding with both bikes, the VFR used exactly double the gas the Yamaha did. We switched bikes back and forth, so riding style is not really an issue.
    Anyway, if you have any thoughts, please let me know.
    Thanks in advance.
    Ken
     
  2. ride

    ride New Member

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    Just an update. Did some more road testing. Seems the vibration is NOT simply at 7000 rpms, but rather at 140+ km/hr (which is close to the time the engine hits 7000 rpms). Doesn't matter what gear, the vibration becomes very noticeable at 140+. (it's evident at slower speeds, but not really an issue).
    Anyway, if you know what might cause a vibration at speed, please let me know.
    BTW, if I pull in the clutch, all is smooth. Must be the drivetrain somewhere.....
     
  3. jethro911

    jethro911 Member

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    This is just a wild guess but you may have a stretched chain. I've seen chains that are only stretched in one area so they go tight and then loose as you rotate the rear wheel. It causes an oscillation in the frame of the bike and is generally noisy as well. Put the bike up on the center stand and rotate the rear wheel by hand to see if the chain slack is consistent or if it tightens up at one point.

    Good luck!
     
  4. ride

    ride New Member

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    Thanks for the input. I don't have a centre stand, but I do plan on replacing the chain in the near future. Seems odd that the vibration becomes considerably more noticeable right around 140km/hr.
    Anyway, any other imput would be appreciated. I'll post a response as soon as I replace the chain.
    Thanks again.
    Ken
     
  5. jethro911

    jethro911 Member

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    Vibration is a complicated thing to diagnose. I've done quite a lot of vibration analysis and balancing on helicopters and aircraft propellors. You need to eliminate options! Ask the question, is this associated with engine rpm or road speed? More than likely it is a combination of both so it is a harmonic where two sources create a bump in the amplitude at the point where the frequencies match. In your case it seems to happen at a certain speed. The nice thing is that you only need to fix or alter one source of vibration to greatly reduce the vibration you are feeling or eliminate the harmonic. Ideally you would like to reduce both but one will give you results. Tire balance and or tire carcass issues can show up at certain speeds as well so don't write off the basic simple stuff that you can eliminate cheaply.:peace:
     
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