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Thread: 76 R90/6 - ideal RPM's & engine braking

  1. #16
    Registered User VIEJO's Avatar
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    Quote Originally Posted by VIEJO View Post
    Welcome!

    My first BMW was a '76 R90/6.... I bought it new, put about 200K on it and regret ever parting with it. In retrospect it was probably the best bike I've ever owned.

    As noted in one of the responses below the '76 has several good features not found on the earlier /6's. One of these features (also on the '75 /6's I believe) is that the cylinders have an extra longitudinal boss in the cooling fins which enables the cylinder to accept a bore out to 1000cc. I did this to my bike at about 60K to take out the slight high end buzz that most of the 900cc bikes experienced. I had the new pistons teflon coated before installing and never had any trouble with the conversion. I stayed with the stock Bing carb's but added a set of slightly less restrictive megaphones from D&D in Haltom City, TX., which was like putting a slightly higher duration cam in..... greatly increased the throttle response. The only other thing I would consider would be a /7 front end to get the twin disc brake setup.

    Congratulations.... you've got one of the best!
    A couple of additions to the above......

    I second the motion that you link up with the Airheads Beemer Club ASAP.... incredible resource for information/guidance/folly on all things Airhead related. In all probability every question you may have about Airheads has been answered there.

    With regards to your question(s) about shifting in general. Airheads are characterized by a rather clunky shifting feel.... nothing amiss, just the result of fairly heavy flywheels (I do not subscribe to the "lighten the flywheels" school of thought). Most Airhead riders I know will "pre-load" the shifter slightly just prior to shifting, up or down. Not a great deal of pressure, only enough to take the slack out of the shifter. If you decide to do this it will become second nature quickly..... I do it on my K bike also.

    There is a plethora of aftermarket gear available to bring your Airhead into the 21st Century and most of it is quite good. However, your low milage '76 is quite capable of running for many miles/years with the gear originally fitted. Don't be in a headlong rush to start "modernizing" unless you have deep pockets and the money's burning a hole in them. If the R90 series had a weak spot it was the ignition/charging system; good for the time but still room for improvement. Consider a modern solid state ignition/charging system for an initial upgrade. EME [www.euromotoelectrics.com] is a great resource for all things electric.

  2. #17
    Airhead
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    4K and blip it on downshifts

    On my stock 1975/6 4K is the sweet spot.

    On 1->2 shifts I preload the shifter to avoid the massive 'clunk'. I get it right 7 times out of ten, all occasions for rejoicing. The other 3 times I check the rear view mirrors to see if my gearbox has shed parts. After almost 30 years with the bike this is the best I can manage, having long resigned myself to the belief that the clunk is a feature, not a problem.

    And on all downshifts I rev match by blipping the throttle when the clutch is disengaged. This makes for smooth shifts. Otherwise the heavy stock flywheel slows things down too much and you are looking at earlier clutch plate replacement.

    Enjoy your bike - itís a beauty.

  3. #18
    Registered User VIEJO's Avatar
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    Quote Originally Posted by THOMASPIN View Post
    On my stock 1975/6 4K is the sweet spot.

    On 1->2 shifts I preload the shifter to avoid the massive 'clunk'. I get it right 7 times out of ten, all occasions for rejoicing. The other 3 times I check the rear view mirrors to see if my gearbox has shed parts. After almost 30 years with the bike this is the best I can manage, having long resigned myself to the belief that the clunk is a feature, not a problem.

    And on all downshifts I rev match by blipping the throttle when the clutch is disengaged. This makes for smooth shifts. Otherwise the heavy stock flywheel slows things down too much and you are looking at earlier clutch plate replacement.

    Enjoy your bike - itís a beauty.
    Some further thoughts about downshifting.... if you are downshifting for braking purposes then I would think you would want to make the gear change as quickly as possible in which case the two essentials are knowing the bite point of your clutch and the relative throttle grip rotation required to achieve the desired RPM. IN MOST CASES it isn't really necessary to pull the clutch lever in to full stop or close/open the throttle completely when shifting... up or down. Fine tuning your shift points is just a matter of consistent riding.

    Personally, I do not favor downshifting for braking on a /6. If done properly it's a perfectly safe maneuver... but hard on tires, clutch assembly, u-joint/rear spline assemblies, and ultimately the wallet. Downshifting to match engine rpm with road speed is another matter and is generally done in a more "leisurely" manner than a braking downshift.

    YMMV,

    Viejo

  4. #19
    Registered User tanker4me's Avatar
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    From what I've learned, in stock tune 4000 RPM on an Airhead is a sweet spot for carbon buildup.*
    It was tough for me to do at first, with most of the torque being on the low side of 5000 RPM, but the Airheads I have/owned run cleaner at 5000 RPM and above.
    Max torque on /5&6 bikes is at 5,000 RPM, on my '92 R100GSPD max torque is at 3,750 RPM.

    The PD had a top end reseal at 30,000 mi.
    The mechanic had to decarbonize the piston etc.
    He recommended to run at 5,000 RPM after I told him that I usually run at around 4,000 RPM.
    Recently when the heads were off at 80,000 miles, there wasn't any carbon build up. The same was true on my '76 R90/6 at 70,000 mi.

    Bill

    *Oil getting past rings & valve guides will also cause carbon to buildup on valves, heads & pistons.

    92 GSPD 77,777.jpeg
    Photo is near Sherman Pass Ca. (9,200') 9/7/17
    Last edited by tanker4me; 11-05-2019 at 09:23 PM.
    We are all here for a spell, get all the good laughs you can.
    Will Rogers

  5. #20
    I have a 75 R90/6. I just ride it like every other bike I've owned of the same age. Upshift/ downshift very much like Honda Goldwing from 1975 till 1987.
    Have enjoyed the performance of my stock model. To be honest, I don't baby mine when I ride it, or give it special attention during the ride.

    Great find, have fun.

  6. #21
    Airmarshal-IL James.A's Avatar
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    My experience is that balky down shifting is a sign that your bike needs the transmission input splines re- lubed. It is a part of routine care. Your bike is 40+ years old, and if your odometer reading is true and correct, it may have never been done.

    My advice to new airhead owners is to get used to the 3k or higher RPM base line and use the gears to maintain road speed at that RPM range. For me, in town riding, my R75/5 (owned since 1989) essentially has a 2 speed transmission up to 45 MPH.

    The Airheads Beemer Club is the best resource for owning and riding these magnificent antique motorcycles. Lovely bike, by the way.

    Al the best,
    James A...
    1973 R75/5

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