After reading tons of forum posts and watching hours of youtube videos, I finally bought a 1985 K100RS with 55.000km on it, and now I’m not sure what to do with it. My intention at first was to build a café racer, but the more I learn about the bike, the more I like the original design. I’m at a point where it almost feels disrespectful towards the engineers at BMW to consider messing with the brickalicious stock version. And what would Chris R Harris think??

Anyway, while pondering these big questions, I plan to fix a couple of possible problems; a «knocking» sound coming from somewhere around the gearbox/rear of the engine, and a suspiscious «hissing» noise from somewhere around the fuel tank. Gonna trawl the forum for info, but any hints or tips would be much appreciated!

Welcome along ExcelKebab,

Great choice of bike, good decision not to cafe chop her (at least not until you've given her a chance to show what she can do) and some great K owners in that there country of yours.

Hissing in vicinity of fuel tank - quite likely to be the noise of the fuel pump running (and completely normal) - and that's a real good start to a well running bike. As for the knocking, I've no experience in that department, but some recent posts on the subject which might help, if not I'm sure that someone will provide some guidance.

Enjoy

Thanks Paul!

Welcome. Re knocking sound. Is it pronounced when the motor is cold but disappears when it's warm?  If so it could be the rivets in the output shaft. Not fatal, just somewhat annoying.

Check the inlet tract from the filter to the cylinder head for leaks. This can be done using an unlit propane torch and the gas will be drawn into the engine and a difference in running is noticed. All the clamps above and below the Injector body as well as the rubber stub manifolds at the cyl head are good places to start. If the engine is idling rough then that will cause rattles so check for air leaks first.

Thanks for alle the tips! I think I found the source of the knocking. For some reason, 5-6 teeth on the anti backlash gear were bent outwards, scraping against the aluminium casing of the engine block. Didn't manage to find a replacement anti backlash gear that cost less than an arm and a leg, so the solution was to sand down the outer/rear edges of the teeth so they rotate clear of the engine block. Hoping this will not significantly affect the weight balance of the gear and cause vibrations.

Welcome Excelkebab

Got a bugger on your hand there

Maybe a machine shop with a press might be the answer,What could have caused this to happen in the first place

The front bearing circlip seemed to be out of place when I disassembled the engine, so possibly the whole output shaft could have slid backwards causing the gear to catch the inner wall of the engine block. Or someone forgot a wrench in there at some point.. 

Excellent work to track that down. It's a very unusual thing to happen.

If that is an 8 valve bike you should consider upgrading the output shaft while you are in there to the later one with more rivets, they don't wear as badly as the 6 rivet one.

Oh and Welcome to the forum, good to have you with us!

Welcome to the forum. If you have a picture of the beauty don't hesitate to share. 

I have a similar knocking sound/ feel to what you describe. I never figured it out but also feels like it's coming from the drive shaft. 

Let new know if you want to meet up for a ride in the Oslo area. 

Cheers
Einar.