R90/6 with the shakes

Discussion in 'Airheads' started by Captdan, Dec 1, 2012.

  1. Captdan

    Captdan Been here awhile

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    I just completed a revamp of a r90/6 into a cafe racer. Runs great on the high end but tends to vibrate on the lower end. I've done a carb sync but still have a harmonic vibration when at lower speeds and going through the gears. I'm to the point now that I'm checking drive shaft connection bolts, and have concerns about the lower end of the engine. I did replace the rear main during the conversion and while I torqued all the bolts to spec in the back of my mind I'm wondering if there was something I missed. It could be that it simply road vibrations and nothing more. I did after all change the seat to a racer seat with little to no vibration mounting, changed the shocks to stiffer ones, bolted the strait bars directly to the top triple mounting without rubber vibration dampening etc. Oddly the bike smooths out when running down the road at 65-70 and past 4 k rpm is fine. ( which of course prompts me to ride the darn thing faster than I should )

    I'd appreciate any ideas on what to check next. This is. It my first airhead, but my first r90 ( I've got a r60/2 that is smooth as silk). This r90 however is not,,...
    #1
  2. JonnyCash

    JonnyCash turd polisher

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    Smoothness is very subjective. I have found though, that the R90 and R100 are definitely not as smooth as the smaller boxers. I've got an R100 which runs very well, but I recently rode a friends R60/6 that while gutless, was super smooth in comparison to mine. That said, they do run very nicely when set up right. I wouldn't think that you've got a bottom end problem. Often, if I suspect my crabs are out of synch, I'll reach down while riding and push a little on the exposed throttle cable between the adjuster and the throttle lever on the carb on one side and then the other. Doing this, it's easy to feel the engine get smoother when you effectively tighten the correct side, or rougher when you push the wrong side.

    It's hard sorting out a bike when you don't really have a baseline to compare to. It's really helpful to have a buddy with a fairly similar bike to contrast your triumphs and failures. Good luck! Oh, and post some pictures!
    #2
  3. Captdan

    Captdan Been here awhile

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    Thanks, good points. I did my carb sync again and she still runs like a bat at high rpm, but I still have a vibration at middle rpm and speeds. I think it may indeed just be that I'm trying to compare this r90 with my r60 ( the 60 runs like a Vespa compared to the r90, simply smooooooth ). I've got a friend with a r100. I'm trying to get a time when we can swap bikes, that would tell me a lot. Thanks for the reply.
    #3
  4. Dolly Sod

    Dolly Sod I want to do right, but not right now

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    You're on the right track. Ride another one. I rode my buddies r60 and couldn't even believe the motor was running. Gutless but smooth.

    my R90/6 has the S rear end and doesn't like to be in 5th at anything less than 55 mph. She starts lugging below that.
    #4
  5. Captdan

    Captdan Been here awhile

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    Well first thanks for response and ideas. Yesterday I was able to locte two other local gents that have BMW r100s of the same vintage as my r90. After riding one and comparing another it seems my r90 is doing nothing but the norm. I did find that mine in the present configuration as a cafe racer does add to the road and engine vibrations. Or at least make them more noticeable. And that's fine, I just wanted to know if mine was hosed or not. Seems these r90s and r100s vibrate much more than the older r60 ( cira 1968) I own. It was a great afternoon talking with the guys that love and maintain these older bikes. I learned a lot on other topics as well once we determined that mine was healthy. I've found that I really like the old airheads. In fact I'd rather ride the r90 cafe around town than the r1200 GSA. Just feels like a real motorcycle!

    Thanks to all that posted.


    I'd post a photo of my bike but cannot seem to find a means to do so..... ?
    #5
  6. Bill Harris

    Bill Harris Confirmed Curmudgeon

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  7. disston

    disston ShadeTreeExpert

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    When you first compared the R90 to an R60 I think it would be natural to think of R60/5 or /6. Now we are to understand that all along you were comparing the R90 to an R60/2. These machines have very little in common other than both being made by BMW. The /2s are famous for that velvety smooth performance. The Airheads were a new design in 1970 and not the same. Some of the smaller engines are not as bad as an R90 but none of the Airheads can be said to equal the smoothness of a /2.
    #7
  8. crazydrummerdude

    crazydrummerdude Wacky Bongo Boy

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    Although you've "solved" the problem, I guess I'll contribute.

    I have:
    1974 R60/6,
    1974 R75/6
    1974 R90/6.

    All run. I'd say the R60 and R75 are incredibly smooth, I'd compare them to Lay-Z-Boy's. I'd say my R90 is "buzzier," not too shakey. They'll all shake with the "right" throttle response under different loads (usually lugging), but get in the RPM range they were born for (3750+), and they smooth right out. They're all great bikes. Life is good.

    I also have an R60/2, but it doesn't run, yet. :D
    #8
  9. Captdan

    Captdan Been here awhile

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    In my initial post I indicated r 60/2 was the airhead I was comparing the r90/6 to. It is after all the only other airhead I've had and naturally compared the two. Now I see they are two very different animals in some ways. However I'd not know that until my present quest for knowledge.

    Thanks again for the info and replys
    #9
  10. disston

    disston ShadeTreeExpert

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    Ooops. You're right. I missed this item in your first post.
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  11. advNZer?

    advNZer? Long timer

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    my r100gs is almost dead smooth at 4000 rpm and above,.Its definetly not as smooth below that
    #11
  12. Owly

    Owly Into the Dark

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    They are prone to vibs if the engine mounting bolts are hand tightened
    , check the torque on all the bolts that hold the brummmm bits to the frame :)
    #12
  13. Captdan

    Captdan Been here awhile

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    Well, you made a valid point.... The two are not comparable... Didn't know that, but now I'm educated ( or more so, still learning) .
    #13
  14. Captdan

    Captdan Been here awhile

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    :1drink

    Exactly what I found out. Shakes is a strong word.. More of a vibration. And yes, above 4k she is all out and smooth. I find high gear at 40 mph to be where the worst of the vibration is.. ( so down shift or speed up solves the issue for the most part). After riding the other r100s mine is fine.

    Today I went on an 80 mile blast and noted my rear man repair job didn't work. So, out comes the trans again soon. :evil. I've noticed some oil leaking, remained in denial but after today given the amount of oil leaking from the lower valley and seeping on both sides where the trans and engine block mate something is amiss. Either I hosed the rear main or the oil pump gasket when replacing them. Dang, always something! Still, I really like the bike! Looks great, runs fine and when I get the kinks worked out I'm sure she will be a fine ride.
    #14
  15. disston

    disston ShadeTreeExpert

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    The bottom frame attachment nuts have been mentioned. Owly mentioned them. They have a torque value and it is higher than I would have suspected. It is not in the book, not in my /6 manual anyway. Maybe it's mentioned in Hayne's or Clymer's I don't know but I got this figure from a Guru and I believe it is correct. The frame stud nuts are torqued to 55 ft/lbs.
    #15
  16. Captdan

    Captdan Been here awhile

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    Thanks, I did torque the bolts to 55 lbs. Seems things are normal on the bike. I was not schooled on what was normal and not regarding vibration on a mid 70's r90 and r100.
    #16