85 K100RS intermittent engine rattle

[Andy FitzGibbon]

1985 K100RS, 46k miles.  It's got an intermittent, metallic-sounding engine "rattle". Not a knock- kind of sounds like a bucket of bolts being shaken, or a small team of dwarves in the engine, all tapping with very small hammers.  Been there since I got it running last November.  I can't think of a safe way to get a sound recording of the noise, as it only happens while in motion.


Symptoms:
Manifests itself at part throttle (coasting in gear, ie. 35 mph zones) and when accelerating from below 2500 RPM.  Goes away immediately when accelerating from higher RPMs.  Does not seem to make any noise at idle, or be influenced by whether the engine is hot or cold.


From the reading/searching I've been doing, it sounds like I should look at the following:


Anti-backlash spring
Cush drive rubbers
Output shaft rivets
Alternator drive rubbers
Possibly alternator bearings


Is there anything else I should be considering?


Thanks,
Andy

[Laitch]

Is there anything else I should be considering?

I think you should consider operating with higher revs before you change gear. Most of these engines will rattle mercilessly when lugged. They should get higher revs starting off and higher revs in general.

[Andy FitzGibbon]

I think you should consider operating with higher revs before you change gear. Most of these engines will rattle mercilessly when lugged. They should get higher revs starting off and higher revs in general.

The noise occurs when I'm lugging away from a dead stop in first gear- not much I can do to raise the revs in that situation.  It goes away once 2500-3000 RPM is reached, and stays gone, as long as I am accelerating.  It will re-occur if I start "coasting" along at part throttle, with the drivetrain mostly unloaded, regardless of engine RPM.  Once I accelerate from that situation, and load the drivetrain, it will disappear once again.


It's an interesting problem, because it seems to be caused by drivetrain loading at low RPM, and by drivetrain unloading at high RPM.  Of course, there could be two different problems that create a similar sound.


Andy

[Laitch]

The noise occurs when I'm lugging away from a dead stop in first gear- not much I can do to raise the revs in that situation.

There is something you can do and you'll either learn it or you won't. It's unnecessary to rattle away from every stop if it's the typical K-rattle. Consider strapping a mic to the inside of one of the battery covers and recording from there. I just learned recently that sharing is caring.  :yes

On the other hand, I think you might feel more confident if you just replace everything that's on your list.

[Andy FitzGibbon]

Is this a way of describing cruising at a steady rpm, Andy? It shouldn't rattle like a bucket of bolts then, that's for certain. Otherwise, I don't understand what you're describing and why you're running like that. Coasting to me means rolling when out of gear. Decelerating means slowing down in gear. There should be no b-o-b rattle in those conditions either.

Probably a better way to explain it is that rattling occurs under the following conditions:


1. Acceleration from a dead stop or low RPM, up to about 2500 RPM.  At that point, it disappears, as long as acceleration load on the drivetrain is maintained.


2. Coasting/cruising, IN GEAR (drivetrain unloaded) at any RPM (e.g. cruising along in a 25 MPH zone on flat road). Loading the drivetrain by accelerating makes it disappear again- as long as the engine speed is above 2500 RPM. Loading the drivetrain by decelerating seems to improve things, also, though it's harder to tell due to exhaust noise.

[Laitch]

You're describing incorrect gear choices to me, so far. From a standing start you need to be raising the revs higher and smoothly releasing the clutch into those revs. It takes some coordination.

At 25 mph you should be in second or third gear as you cruise along. If you're in fourth and try to accelerate rapidly, your engine will probably rattle. It might rattle when in third, too, if you give a hard twist to the throttle.

[Andy FitzGibbon]

You're describing incorrect gear choices to me, so far. From a standing start you need to be raising the revs higher and smoothly releasing the clutch into those revs. It takes some coordination.

This is exactly what I do.  The engine rattles from a standing start UP TO 2500-3000 RPM, at which point it goes away, regardless of whether the clutch is slipped or suddenly released.

Do you rev your bike to 3000 RPM and then slip the clutch in?  That seems wasteful and excessive to me.

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At 25 mph you should be in second or third gear as you cruise along. If you're in fourth and try to accelerate rapidly, your engine will probably rattle. It might rattle when in third, too, if you give a hard twist to the throttle.

I run in 2nd gear at 25 MPH, or sometimes 3rd. If I'm in 3rd, and have to accelerate, I'll shift to 2nd before doing so. 


As I said before, when cruising, the noise GOES AWAY when accelerating, as long as the engine speed is 2500 or greater.  The noise is heard when the drivetrain is unloaded/lightly loaded, in gear, no matter what the engine RPM is.


I understand that the engine will have some inherent rattle when lugged.  But this rattle seems different to me, particularly for the fact that it also occurs when the engine ISN'T being lugged.  Since the bike is an '85 model, it should theoretically have the six-rivet output gear and the early-style anti-backlash spring, both of which were subsequently updated due to failures and excessive noise issues.  Hence, my original question: mechanical problems I should be considering besides those that I listed.


Andy

[rbm]

No, you shouldn't rev to 3000 and then slip the clutch. That will just wear it out in short order.


The list of items you gave in the first post are the most likely suspects.  Being an older K, it's possible that the 6 rivet output shaft  is worn and needs attention.  If you get an opportunity, try and obtain a 12 rivet output shaft from a later model.

[Laitch]

Do you rev your bike to 3000 RPM and then slip the clutch in?  That seems wasteful and excessive to me.

Nope. I don't do that but it did rattle until I learned to shift differently.

I guess we're back to square one, if we ever left it. :giggles

[Laitch]

Nope

Being an older K, it's possible that the 6 rivet output shaft  is worn and needs attention.  If you get an opportunity, try and obtain a 12 rivet output shaft from a later model.

I like that suggestion—those '85s and their imperfect parts. Scroll down to Time for the engine inspection in this document to see the part and other possible offenders.

[Andy FitzGibbon]

No, you shouldn't rev to 3000 and then slip the clutch. That will just wear it out in short order.


The list of items you gave in the first post are the most likely suspects.  Being an older K, it's possible that the 6 rivet output shaft  is worn and needs attention.  If you get an opportunity, try and obtain a 12 rivet output shaft from a later model.

Thank you- that's exactly the sort of information I was looking for.


Andy

[rbm]

Please refer to this document if you discover that the culprit is a worn output shaft and you decide to repair the output shaft instead of replace it. The text is German but a little time spent with Goggle Translate will reap legibility.

[BrickMW]

I have this same noise / feeling. I say feeling because to me it reports more to my sense of touch rather than hearing a specific knock / grind / clank.


You have done a great job at describing this condition Andy, which I have found difficult to accurately do, therefore I have forgone attempts. My research has pointed to the same things you have on your punch list and I will be doing a pretty intensive tear down this fall to inspect clutch/tranny/alt/etc... meanwhile The Mighty Gryphon has me optimistic that it is just my K75 balance shaft, and no real concern as THIS (see reply #2) is also a pretty fitting description of the anomaly. (fingers crossed) This wouldn't fit with the K100 though, it being sans balance shaft, so I am skeptical since your description is so fitting.


A general question I do have for my fellow motobrickers... How much rotational lash is present at the rear wheel when in gear? i.e.--Moto on center stand, in first gear, engine off, how far will rear wheel rotate freely back and forth as measured, say at the valve stem? Mine seems excessive considering the condition of my rear splines.

[The Mighty Gryphon]

Regarding rear wheel lash.  In 2nd gear I have eyeballed the lash at about 1/2" at the valve stem on my K75's, and my K100 has about 1/2" to 3/4".

The K75's have 82k and 40k miles.  The K100 has a Paralever and about 122k miles.  All three bikes have little or no visible spline wear. 

I believe there is little to no lash in the final drive or drive shaft on my bikes.  My impression is that most of whatever lash there is seems to come from the transmission, probably the dogs on the gears.