Hi there,
I am new in the forum and got recently an k100 1985.

The bike does a rattle noise, especially if i rev it to 3/4000 and let if off... no strange noise at idle... but is very pronounced when the engine is "desacelarating".

I have removed the alternator and put the bike to work... rattle still there, so no alternator. (but just cant find the minimum dimension for the monkey nuts... as i feel they might need also to be changed)

The clutch on or off doesnt make any diference.

Any pointers?
Final drive? 

Thanks for you help, as i am sooooo excited to have it running smooth for some summer rides 

Is the noise different if you remove the fill plug for the final drive, gear box,  engine,  or petrol tank? Block one ear while trying with the other to pinpoint the noise to its location.  Regular? high pitch?

Different in neutral or gear while on centre stand?

GK100 wrote:I . . . got recently an k100 1985.
The bike does a rattle noise, especially if i rev it to 3/4000 and let if off...Any pointers?

The noise could be from a defective fuel return valve. Locating it is relatively easy. Read this entire thread then locate the valve and eliminate it as a possibility.

When engine noise is involved, it is helpful to post a sound file or YouTube video recording of the noise.

I too had a strange rattle on shutting down,turned out to be the heat shield on the muffler.When you rev it on the stand put your foot on the shield if the rattle stops youv got it .Hope its a easy fix for you anyway.Im finding that most probleme can be over thought at times

Two more things come to mind.  The lower end of the clutch cable is attached to an arm that if there is too much slack can rattle on the muffler.  Another source can possibly be a rattle in the output shaft.  The rivets on early model K100 output shaft dampers can loosen up.  Many older K100 engines have had the old 6 rivet shafts replaced with 12 rivet shafts.

Thanks very much for the pointers!!

I have done a video and uploaded into youtube https://youtu.be/zBGS6td4lhI dispite the rattle noise doesnt seem that serious on the video, seems a lot louder in reality (might try to make a new one...)

Tried also with the to pinpoint more accuratly were it come from, its hard because it ressonates the entire bike, but seems to come from the right side of the bike.

I have removed the mufler cover, and all seems secure.

Goint to try next taking the fill bolts and see if the noise is different

So far seems the same being on center stand or on wheels, clutch pressed with gear or neutral... 


Point-Seven-five - The rivets on early model K100 output shaft dampers can loosen up.  Many older K100 engines have had the old 6 rivet shafts replaced with 12 rivet shafts. -  How do i check this?

To check the rivets, you need to get into the bottom of the engine.  It is not a job you want to do until you have checked everything else.

Either buy an automotive stethoscope or use a large screwdriver. Place the blade on the suspected areas with your ear pressed against the handle, Move it around until you find the source of the noise.
Regards Martin.

Is the noise reduced after the say 20 minutes, when the engine is warm? I had a noise similar to that, maybe its a tad more metallic, in one of my bricks caused by the rubber cush drives. Apparently they get too hard over the years becoming noisy (opposite to what i would have thought, that rubber would go softer over time) After the other options are exhausted, you can see most, if not all of the output rivets by removing the right side engine cover (the one with the oil fill cap)

On mine, when the engine and cush drives were warm, the noise was reduced, but only by a little.

how many km. has your bike travelled and does the odometer work?

Thanks again for all the help,

The bike has 120.000kms and the odometer works.

To check the rubber cush drives i have to remove the back wheel?

Am going to buy an automotive stethoscope, not having much luck with the screwdriver.

I had plans to do a full maintenence since i got the bike recently and has no history with it, and hopefully during the way will stumble with the issue (optimistic!!). So first will try and sort out the filters, oils... and see how it evolves.

To check the rubber cush drives i have to remove the back wheel?

GK100 wrote:I have removed the alternator and put the bike to work... rattle still there, so no alternator. (but just cant find the minimum dimension for the monkey nuts... as i feel they might need also to be changed)

GK100 wrote:To check the rubber cush drives i have to remove the back wheel?

The drive cushions are part of the alternator assembly so if you removed the alternator, you had access to them right then. Monkey nuts is the nickname for the drive cushions. You won't find Monkey nuts listed in a BMW parts fiche. Your first post implies that you viewed them already.

Did you check to see if your fuel tank's fuel return valve was making noise?

Laitch wrote:

GK100 wrote:I have removed the alternator and put the bike to work... rattle still there, so no alternator. (but just cant find the minimum dimension for the monkey nuts... as i feel they might need also to be changed)

GK100 wrote:To check the rubber cush drives i have to remove the back wheel?

The drive cushions are part of the alternator assembly so if you removed the alternator, you had access to them right then. Monkey nuts is the nickname for the drive cushions. You won't find Monkey nuts listed in a BMW parts fiche. Your first post implies that you viewed them already.

Did you check to see if your fuel tank's fuel return valve was making noise?

Any time I took out an alternator the monkey nuts fell out too.

Sorry I botched the description of the problem I had, which was caused by hardened rubber dampers in the output shaft. If that's your problem it's a non urgent fix, but I'm not confident as I'm thinking my noise was a little deeper in pitch.

Anyway set that aside and do the easy stuff first. What are the results of the previous suggestions you havn't reported yet? 

While waiting for stethoscope get a Clymer manual,  a late edition Haynes, and the free manual from this site With "All 2V models" in the title  (look at pages 11-13 onwards) 

Keep trying to pinpoint location, water pump, timing cover, housing between engine - gearbox higher or lower, sump etc. Is noise reduced when warm, etc?

Maybe ride to a mechanic,  ideally BMW, for a diagnosis and quote.

Can we see a left side photo of the bike?

I am goint to do some work in the bike in a couple days, got an stethoscope and i have the Haynes (should try to get Clymer as well... lets see)

So far no more news, just too much of a slag on the back wheel i noticed (on gear and central stand... i feel its too much)

Sorry, with "rubber cush drives" i thought it was something else... too many names for the monkey nuts!! 

Did went with it to a mechanic before starting to do anything, he said it should be alternator or clutch, but since the noise doesnt change with clutch position or been in gear i thought that it shouldnt be it...

I also did listen with the screwdriver to the fuel return valve and it sounded ok...

Still scraching my head with this, but once again thanks for all the help!

Remove the fuel cap anyway as a sound check.  Also to check for fuel hose, float,  or something tapping.  Remove the alternator drive too.  Hopefully it is related to the clutch,  we are hoping for the cause to be outside the engine. The clutch is still a possibility as most components are still turning even with the clutch pulled.

Do make sure you know if the noise changes with engine speed in neutral.....but also run it on centre stand in gear and change gears.

However......my thought is look in the water pump, they do strange things if left sitting unused for aeons as yours has done and without coolant [just water] and seen some fairly disintegrated or breaking up impellers.

Hi there,

Have done a bit of work... and the water pump is not good... going to try to sort it and repair it.

But i think thats still not were the noise came from, and since the bike is now in parts not easy to try just with a new one, so i think i am going to investigate a bit deeper. 


I will try to keep you posted with news!