K75 Engine output shaft noise

[tricyclerob]

On the IBMWR there are 2 good tech articles on output shaft noise.

In the second article there is a photo of the old style and the "new" style driven gears.

http://www.ibmwr.org/ktech/output-shaft-diagnosis.shtml

Does anyone know when [what year] the old style was stopped, and BMW began using the new style?

Thanks, rj

 

[bmwwrench]

The updated output shaft only applies to the 4-cylinder bikes. Their output gear was riveted to an aluminum cush-drive hub, which the K75 does not have.

 

[tricyclerob]

L.B.,
Maybe I'm being dense here, [a distinct possibility]. So the K75 does not have the cush drive at all, and so no rivets to come loose?
I guess if this is so it's a bit of a relief, but now I need to figure out where the vibration is coming from. I saw there was a mention of alt.bearings, so I will check that next.

Actually I was looking at the output shaft drawing for the K75 and the K100 at Real OEM, and I don't see the cush drive on either? Or I'll put it this way, I don't see a difference between the 2.

Am I just not seeing it?
I'm not usually this dense, I think...
thanks, rj

 

[ted]

Might want to replace the monkey nutz while you are in there, they turn to mud over time producing a distinct rattle at idle. When I replaced mine on my K75S they were so far gone the clutch housing (#7 below) fins were chipped and it needed replacing as well (sounds expensive, it isn't.) See #9 in this pic, you need three:

 

[dbrick]

Isn't there an intermediate gear in the 3-cylinder bikes that vibrates with age, as its internal spring (which holds the two gear segments in tension) weakens? I remember this noise from my K, which had 66K at the time.

(pause)

Here's a mention:

On the K75 bikes, there is a spring which provides tension between two gears on the intermediate shaft. If this spring gets soft, the motor will buzz at lower rpms. This is moderately common; the motor will run happily anyway.

 

[deilenberger]

David - same thing on the K100 bikes. One gear has a secondary thin gear attached with a horseshoe shaped spring that is meant to keep it in tension on the mating gear. It's called the "anti-rattle" spring.

Problem is - the spring weakens up, and then the secondary thin gear starts rattling. The anti-rattle-gear/spring ends up rattling. BMW revised the design 3 times I know of to solve this.. AFAIK - any brick engine K bike is prone to the problem, even the last ones made. It may be worse on the K75 since the firing cycles are further apart then on the 4 cylinder..

 

[tricyclerob]

So my take is, if it's not the alt., it's probably the "anti-rattle spring", [great name] which is irritating, but won't leave me on the side of the road? Bike is at 44,000 by the way.
Is this what's known the "K-bike Rattle" I've heard mentioned?

Thanks, rj

 

[bmwwrench]

When do you hear the noise? The anti-lash spring gets worked the hardest at low engine speeds.

 

[tricyclerob]

L.B.,
It is most noticable from 1200-1500 up to maybe 2500 rpm, then seems to stop almost abruptly, but I'd probably need to ride again to double check.
I did take the alt. cover off tonight and did not feel any noticable or unusual vibration from the alternator.

There is also some vibration that seems throughout the rpm range,[but not as noticable], almost like it's 2 different things going on.

I think I've been spoiled as about a year ago I was lucky enough to come across a 92 K75S, with 4000 miles on it. $2750 with bags, a Corbin and a complete low seat. The deal you always hear about someone else finding.
The S rides smooth as silk, then I get on the RT [also a 92, but with 40,000+ miles] and I notice every little thing.

If it's just age [a weak spring] and isn't going to leave me on the side of the road, guess I can live with it, but the feeling that somethings not right really bugs me.
On the other hand disassembling 1/2 the bike to fix a weak spring seems like quite the PIA.
rj

 

[bmwwrench]

This is a noise common to K75s. You can reduce it with careful tuning-check synchronization of the throttle bodies and ignition timing, as well as proper mixture.

 

[dbrick]

It is most noticable from 1200-1500 up to maybe 2500 rpm, then seems to stop almost abruptly.

That's exactly what I remember from mine. The conventional wisdom was that the soft spring didn't hurt anything, and wouldn't damage the engine.

I just kept the revs up.

 

[tricyclerob]

That's what I was hoping to hear. I think I'll live with it until I catch up w/some other m/c related projects.
Could be years.
rj

 

[lrome52]

That's what I was hoping to hear. I think I'll live with it until I catch up w/some other m/c related projects.
Could be years.
rj

New anti rattle spring-$5.79, cost to replace-$1000, low RPM 'Growl'-Priceless!