I have a 1985 K100 that I bought a few weeks ago. It seems to run well, is powerful and smooth and the engine is quiet at idle, acceleration and cruising speed. When I decelerate it makes a rattling knocking sound and as soon as I put a bit of power to it the noise stops. I saw a post about this from a guy from Europe a few years ago but haven't been able to contact him. The manual says that noise would be rod bearings. Has anyone else had this problem and? Can you change the rod bearings without removing the motor? Any help would be appreciated.
Thanks, DOug

Hi Doug,

I would not jump in too deep just yet as there are a number of unusual noises these bikes can make.
Remember it has a dry clutch and therefore more noisy than a wet one.
Cam chain tensioners need to be replaced at certain intervals along with the chain.
The alternator drive has rubber inserts that wear and I think in some cases the alternator itself can be noisy.
There is even a non-return valve in the fuel line of early models that can cause concerning noises.
So the short answer is yes I believe you can change the bearings in situ but it would be worth looking elsewhere first.

Regards,

Phil

It seems to me that rod bearings, due to clearance issues from wear, or incorrect fitment, would make noise at idle as much as when under load since oil pressure would be at its lowest. They often come off as a soft knock at idle which increases in tempo with rpm.

Grab a large screwdriver and put your ear to the handle and the tip to the engine and gearbox in various places until it's clearer where the noises are coming from.

I have learnt that the problem is usually the simplest (but not always) and that a little bit of time spent eliminating possibilities is time well spent.

I had an old Airhead that made a rattling noise when decelerating and only in top gear. It went on like that for years (and 80,000 miles) until I had the money and time to repair it. The fifth gear cog was worn and 'wobbling' slightly on the output shaft.

Please let us know what the outcome is, of course.

Thanks for the responses. I'll try to see where the sound is coming from. I have read posts about the cam chain and alternator noises but the sounds in my bike are only as the motor decelerates. At idle it sounds smooth and quiet and under load and at speed the same but only when I release the throttle the noise is obvious. Even at idle if I blip the throttle the noise occurs as soon as I release the throttle.
Thanks, Doug

As yours is an older bike I would consider the rivets in the intermediate shaft vibration damper with the spring loaded gear behind it. These bike take quite some abuse, run for miles and bearings are not a known issue (not that they won't go, but its not a starting point).
Alternatively air leaks can cause strange mechanical like noises as well. What's the history of the bike and what have you done so far?

I agree with Rene about the output shaft rivets. This is a well documented failure. Here is a quick way to check them.

Bert

I bought the bike a few weeks ago about three hours from where I live. A fellow biker friend looked at it for me and heard it run but didn't ride it since it wasn't insured. I have all of the service records since it was new and has had anything done to it that the dealers recommended.
ReneZ, you asked what I have done to it so far. So far I have ridden it home which is about a four hour ride. I am still waiting on collector plates for it so I haven't taken it out again. I am going to take it up the road today to warm it up and then try to see if I can identify the location of the rattle. As I said the only time you can hear it is when you back off the throttle.
I read a thread from a few years back from a guy in Poland who had the same sound and did the alternator and cam change fixes but to no avail. He was pulling out the motor but then never posted again or answered my email.
Would the loose output shaft rivets make the sound that I have described and if so can the damper be replaced without pulling the whole motor apart?
Thanks for the help so far, it is super helpful to have a community of fellow bike freaks to share their experience. The one thing I am convinced of is that this is a great bike. Just the four hour ride home was sooooo nice. I love my XS 650 and will probably never sell it but it will be used for riding around town and not on too may trips.
I'll keep you posted. Thanks, Doug

Doug, has the bike been standing for quite some time? I would first do the basics, check the clutch/throttle/'choke' adjustment, valve clearancess and adjustment/synchronisation of the throttle plates with TPS, idle revs, clean oil (crankcase/gearbox/final drive), new petrol. Check the alternator rubbers and clean all (!) the electrical connectors you can find, but take particular care of the ground points under the tank and on the left side of the intermediate flange between engine and gearbox. Yhen take it for a long spin and see if you still have the same issues. If you want to check for the rivets; remove the righthand crankcase cover (you should be able to do this with the coolingwater hose in place) and look at the gearwheel rivets at the aft end. There is a picture on a site somewhere, If I can find it again I'll post a link.

Here from one of the other great sites.

Man, Bert, you're fast! And you're right, didn't know anymore where I had seen it whilst trolling the fora. Good one!

Well, I just warmed the bike up and the noise seems to be coming from the back of the engine on the shaft side and is noticeable as I rev it up slowly in neutral and it hits about 2,500 to 3,000 RPM. Holding that RPM it is noisy and also as I back off the throttle. Is that the area of the cam chain tensioner or more likely the rivets? Can the loose output shaft rivets be addressed without pulling the whole motor apart?
Sorry for so many questions but I am hoping to be informed before I start attacking the problem or ordering parts.
Thanks again, Doug

Can you do something?

Remove the alternator and run the bike on battery. You will not damage anything. From my personal experience, the alternator could be very noisy and at only certain RPM.
I had open my engine because of the same problem and it turned out being the alternator.
My twin brother (Yes there are 2 like me and he own a K100rt too.) had a similar problem. The engine sounded like a rod bearing was finished. We checked the rods and couldn't find anything. At the end he took the bike to a mechanic who found in 2 min that the alternator was at fault. When I visited him, he had the engine already open and as we live 1,300 km apart, I couldn't hear the noise.

One other problem could be the bearing of the alternator drive.
Check my K100 rebuilt page and you will see which one I am talking about.

To replace the output shaft, you have to remove the engine.

Bert

Thanks again. I will try that and let you know what happens. I am leaving for a week on Monday so I might not get to it right away but Iwill post what I find.
Doug

Hi Doug,

The cam chain tensioner is on the front under the large cover under the radiator so it is starting to sound alternator-ish but noises are dreadful to try and find so lets hope it is something simple.
Just be aware that while relatively easy to remove the inboard alternator bolt can be a little difficult to get a straight purchase on so make sure you use a socket ended allen drive and try and clean out the end of the bolt so we don't strip anything

Regards,

Phil

So do I pull off the Alternator according to the shop manual by removing the fuel injection control unit, coolant recovery tank etc and then put it all back together without the alternator and fire it up?
Doug

I have been away for a few weeks but soon want to get going at fixing my rattle noise. I want to try removing the alternator as was suggested but thought it might be easier to try and check the retunr line valve in the gas tank. Several people mentioned that it might be the cause Is it hard to get at?
Thanks again, Doug

It's easy to fix, but after you remove the valve, the gas will poor from the return side of the tank each time that you remove the hose.
To solve the problem, I installed a small inline tap. I used one sold for snow blowers (They are cheap and resist to heat).

To remove the ball, empty you tank, remove the 'cartridge' on the return side of the tank. Pop up the small washer on the top of the valve and remove the spring and the ball.

I went to a see a guy that had a stethoscope and he says for sure my problem is the alternator bearing. I told him that was the consensus from the forum but I guess i was hoping it would be the return valve. Easier, but oh well. Can just the bearings be replaced or am I better off to replace the alternator? Any ideas on the cost and where is the best place to but parts?
Thanks again. I hope as I gain knowledge I can help others out the way this forum has helped me
Doug

I've had my k100RS down to the bits and pieces. I had the alternator out when the motor was out, but it seemed to me the alternator was not a difficult job even with the motor in. I'm guessing that the bearings are going to be an easy replacement for any auto electrical place. Do you have a good little shop like that in PR? There's a great one out my way in the Fraser Valley, little family owned biz, not like that's very helpful tho...

We do have a shop here that sells bearings so I'll take it apart and look for them. I'm going away for 8 days but will leave a post as to the success of the operation.

Thanks again, Doug

Had my alternator overhauled for about $250 AUD but that did include one bearing costing $60!! but definitely not that difficult to remove.

As for the stethoscope my old CX500 did a cam chain tensioner and the mechanic with the above told me it was a broken crank!

Hi Phil,
I suggest you never ever use that oaf (I refuse to call him a mechanic) again. That is such a wrong diagnosis on so many levels it is ridiculous of him to even suggest it. Sorry for the rant but guys like that enrage me because they cause good people needless expense and worry.
Regards,
K-BIKE

Thankfully Jeremy that was a long time ago etc etc and I was reasonably aware that was an unlikely state of events hence he did not end up repairing it.
The guys that did only put it in backwards!! of which I was unaware until it broke again but that was many thousands of kilometres later and at least it enabled me to get back to Wagga from Melbourne last thing on a very wet long weekend. Had it come unstuck on the way back is another story and just one of many "what ifs"