Planning ahead...R90/6 gearbox Q's...

Discussion in 'Airheads' started by DiabloADV, May 1, 2012.

  1. DiabloADV

    DiabloADV Semi-Occasional

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    I just picked up a '74 R90/6. It runs and shifts terrific. Only issue is that neutral is tricky to find. Letting the clutch out, the gearbox lets out a rattle until I fiddle with the shifter a bit till it's all the way in neutral. I have read that this is a common glitch of this gearbox.

    The only other things I can find about the '74 5-speed is weak pawl spring and weak kickstart gears.

    There is a transmission tech day coming up so I'm wondering whether I should break this box open and go through it, or leave well enough alone. I opened up my /5 4-speed box and everything was in such great shape I just sealed it back up. Any advice on the /6 box? Mileage on the bike is unknown, but it looks like it's been cared for.
    #1
  2. Wirespokes

    Wirespokes Beemerholics Anonymous

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    The rattly noise is common. With familiarity, you'll find neutral rather than being almost there and rattling the dogs against the mating gear.

    If the box lasted this long, it's fine. It's true, they had weaknesses, but quite possibly the box has already been opened and upgraded.

    Some of the early boxes failed from rough use, so if you're not racing, motocrossing or drag racing, you should be fine. I sold a 74 R90 to a woman friend who's had it ten years now. She rides it hard and it's still going strong.

    Don't worry about it!
    #2
  3. DiabloADV

    DiabloADV Semi-Occasional

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    Thanks for that advice. I just got back from a more extended ride and this thing shifts like butter. As opposed to the box of rocks on my /5. ;-)

    I'm going to ride it for the summer and do some deferred maintenance next winter (pushrod seals, etc.).
    #3
  4. wirewrkr

    wirewrkr the thread-killer

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    Yeah, as Wirespokes inferred, if it ain't broke, don't fix it.
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  5. disston

    disston ShadeTreeExpert

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    The pawl spring you heard about is not a '74 year problem but was a 5 speed problem till the updated shift kit. So it is also a '74 problem. But I don't think it has to break. Could have been replaced or up dated on this box and your report about it shifting fine now I think it's OK to leave it alone.

    The advice about not using the kick starter unless absolutely necessary is good advice. Some will even push start the bike to avoid using the kick starter. I had one of those years ago. I used the kick starter sometimes till it broke. it wouldn't work anymore but I think I've heard they can brake in different modes and sometimes cause more damage than just not working.

    The electric starter works just fine. Keep the battery in good condition and you don't need the kick starter.
    #5
  6. AntonLargiader

    AntonLargiader Long timer

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    The hard-to-find-neutral issue is common to early boxes. The shift kit will address this also, but you can also find the parts that came before the shift kit and fix it.

    '74 and '75 - neutral 'bump' for switch
    '76 to '80 - neutral detent for switch
    '81 -> on - neutral detent with improved pawl design (shift kit)

    So if you replace the 3-4 cam with a later one (doesn't have to be shift kit) and change the switch to the corresponding one, you get better neutral finding.

    Pix here: http://www.largiader.com/tech/airtrans/
    #6
  7. DiabloADV

    DiabloADV Semi-Occasional

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    Is there any fix for the weak kicker gears?
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  8. wirewrkr

    wirewrkr the thread-killer

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    Don't use them is the best advice I can offer.
    They were poorly designed and have always been a source of problems when used alot.
    #8
  9. chasbmw

    chasbmw Long timer

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    BMW gearboxes are surprisingly sensitive to clutch adjustment. To do this use, the handlebar adjuster to ensure that the distance from the rear of the crankcase to the front of the clutch rod is 201mm. Then use the adjuster bolt on the clutch arm to ensure that there is 2mm free play at the clutch lever on the handlebars. It's a strange way of doing it, but makes sure that the clutch arm is at the correct angle to operate the clutch pushrod properly.
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  10. Wirespokes

    Wirespokes Beemerholics Anonymous

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    The clutch will work just fine however it's adjusted - as long as there's a little bit of slack at the lever. The reason for the adjustment as you've detailed is for leverage reasons; that's where you'll get the easiest pull - when the clutch arm is parallel to the case at half throw.
    #10
  11. chasbmw

    chasbmw Long timer

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    Wirespokes, On both my bikes, i.e on both heavy and light flywheel models, I find that gear change quality is very closely correlated with clutch adjustment the 201mm method, if the clutch is adjusted properly, then gear changes, finding neutral at stop lights etc etc is very much better, if the clutch is not adjusted properly then I find myself crashing through the gearbox, struggling to find neutral.

    I would assume that this is because the angle of the clutch arm will affect how the clutch arm engages with clutch pushrod..

    In any event , a very careful clutch adjustment is a quick and easy thing to do, before searching further for problems with BMW gear change quality.

    iMHO etc etc
    #11
  12. Wirespokes

    Wirespokes Beemerholics Anonymous

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    That's interesting data chasbmw. It doesn't logically make sense because no matter where the arm is, it should still move the throw-out assembly the same distance.

    However, I can see that if the arm isn't parallel half way through its throw, it'll be putting a sideways force on the throw-out bearing which might possibly bind the assembly in the bore slightly. Properly adjusted, it'll be more of a straight-on drive.
    #12