arrived this morning , got them both running after my introduction to battery access , the blue bike has a knocking noise coming from the clutch , drive shaft area which disappears when a few revs are applied , otherwise the engine is smooth and doesnt blow any blue smoke . the red bike also runs smooth but blows quite a bit of brown smoke , especially when revved  . the photo,s make both bikes appear in better condition than they really are , but im happy with them .

Nice start to aa collection

The're beautiful... but I am biased 

Your stress level will be going up when you get them both running. "Which one today, which one today?"

The knock in the blue one, is it sort of a metallic clank around 1500rpm? If it is, it's just the dog that drives the balance shaft slopping around in the the output shaft gear. Some of them do it. My K75 has been doing it for 45,000 miles. It's part of the bike's "charm".

If it doesn't clank but is more of a muffled rattle, it could be the rubber alternator drive cushions A.K.A. "monkey knutz". They are a wear item and when the get worn they allow the alternator drive dog to rattle in the drive clutch. It's about an afternoon job to replace them.

K75S is the most beautyful K


Check the rubbers of the monkeynuts (in between the atlernator and engine) and the Z-hose (engine to airfilterbox).

Thanks guys , ill look into that , the bikes came with some rubber bushing type things  along with a haynes workshop manual and a BMW training centre " technical features in detail " manual . so ive got a bit of light reading to do , lol

Hey they look very nice, I can see a test ride from Nannup to Bailingup in the not too distant future....
Cheers
Mike

Hi,

If the above suggestions don't work, there are a few other things to try.

If you look at the alternator monkey knuts, they might look undamaged but still need replacing because sometimes they shrink,  resulting in a loose fit.

Does the knock go away when you pull the clutch in?

Although I haven't found a thread with this fix yet, it might be possible that a loose clutch nut could result in a knock sound. I could imagine it being your problem since you say it goes away under load revs.

On an old thread that I can't find back, a knocking noise disappeared after the spark plugs were replaced. Since you have two k75's,  you're in a good position to do a bit of swapping.

its definitely coming from the clutch alternator area , and although it goes away with revs its random at idle also . ill be pulling it down for a rebuild at some point soon , so ill get to the bottom of it eventually , hopefully . thanks for the heads up guys , it gives me something to keep a look out for .

Nice Kollektion there!!!

Its a real puzzle going out and having to choose.......I have the same issue so all bike keys live in my jacket pocket so I don't decide until I am at the bikes.

Welcome from the northern realms. Does the noise go away or change when you pull the clutch in?
Regards Martin.

If the sound is a slightly muffled clanking at 1200 to 1800 rpm, go ahead and tear your engine down.  I will bet a box of donuts this is what you're going to find:



The parts were machined that way and there's nothing you can do about it.  I suppose you could weld them together or buy a bunch of balancer shafts and output gears and see if maybe you can find a pair that fits together better.

Like I said above, some of the bikes make that noise, some don't.  It took me a couple years and three weeks working in the garage tearing the engine down to finally accept the noise as part of the bike's charm and that it won't hurt anything(I've had that noise on my K75RT now for well over 40,000 miles).  I had a K75S that also made the noise, but not as loud(same rpm range and clank sound).

You can also fix it by selling your K75 and getting a K100. The output shaft on the K100 does not have that slot and tang arrangement, so they don't make the noise.

well im not going to be a searching for a problem and im not tearing the engine down anymore than necessary to clean , because they both run sweet , but as these bikes have done a lot of k, s   , like what i did there , lol , 160,000k and 180,000k respectively im certainly going to be checking things for wear , particularly the water pump

Seems to be a heck of a lot of slop in that photo; all 3 '75 engines that I've meddled with were a really snug fit. I'd guess that is akin to the six rivet output shaft issue on the '100: the slightest bit of wiggle will eventually become a lot.

chris846 wrote:Seems to be a heck of a lot of slop in that photo; all 3 '75 engines that I've meddled with were a really snug fit. I'd guess that is akin to the six rivet output shaft issue on the '100: the slightest bit of wiggle will eventually become a lot.

I agree, it doesn't seem to align with traditional German ideas of precision. However, very close examination of the parts in question revealed absolutely no sign of wear that could have caused the gap. Since it looks for all the world like the sloppy fit came from the factory, and that a K75S with 34,000 miles I briefly owned also had exactly the same noise, I concluded that it is part of the design and has some sort of esoteric relation to dynamic balancing of the engine. I really have no idea why it's there.

I'm with .75, I'll bet donuts you will find a gap (until reading Chris's post) and that you won't find endfloat of the output shaft.

But since it's new to you, I thought you might do a spline lube soon, so why not check end float while you're there.

The are a number of problems that could result in engine knocking. I'm hoping it's something that doesn't require opening the engine. Spose it's a loose clutch nut, easy to check. Spose the clutch nut thrust washer has broken and fallen away, again fixable without opening the engine.

I'll guess the tabs are not knocking cause there's no marks on the tabs or slots. Also the clutch nut torque on the k75 is over 100 ft/lb, that's way more than the k100. With the main resistance from the pump, I spose, that wouldn't be enough to overcome 100 ft/lb.

I'm also with .75 in not going straight to opening the engine, especially after the epic "output shaft questions" thread by Audibob.

Before you go in check out the injectors as in cleaning/refurbing. A bad injector can cause serious knocking too, causes diesel engines to be incredibly loud and its nicknamed 'dieselising'.