This is in regards to a 1992 K100RS.  So.. after finally figuring out that electrical issue, now I've got an even bigger problem.  It feels like my transmission is dying.  I'll list how each gear rides and sounds.  I put the bike on the center stand with the rear wheel off the ground so I could put it in gear and listen to the transmission.

1st gear: works fine, sounds okay, but could sound better.. kind of a light rumbling
2nd gear: WTF.. if I try to grab any kind of throttle, the bike skips, lurches, makes grinding sounds and metal smacking sounds.. all sorts of bad stuff.  It first started as a little miss / lurch feeling this morning that could have been written off as electrical issues.  Now, it seems undeniably mechanical and coming from the transmission.  It sounds worse than first gear, more rumbling and knocking from the transmission.
3rd gear: works fine, no skipping / lurching, but sounds worse than 2nd in the rumble and knock department
4th gear: works fine, sounding even worse.
5th gear: works fine, still sounds bad.
6th gear: 6th gear seems to have disappeared altogether.

My theory for 2nd gear is that it's popping out of gear on under stress / power, and then back in, repeatedly, causing the lurches.

This transmission never really shifted super easily between first and second, but it never really seemed bad.  It is now sometimes difficult to get it from first in the second, but it still shifts nonetheless.  Shifting between 2, 3, 4, and 5 is still normal (not super smooth, but not necessarily 'un-smooth').

Transmission has 113,000 miles on it.  I've had it since 97,000 miles.  I don't know about the previous owner (it only ran on 3 cylinders when I got it), but I've always kept the transmission oiled correctly etc etc.  Yes, there's oil in it now, and the oil was crystal clean the last time I checked it a couple weeks ago.  No oil leakage from transmission.

I'm worried this is probably only going to get worse.  The fact that the other gears sound bad as well makes me want to look in to a new (probably ebay used or something) transmission altogether and just get it done.

I would post a video of the skipping / sipping / lurching / etc in 2nd gear, but have no means to video it while moving.  I can try to rig something up if anyone thinks it will help.

 I wanna tear in to it right now, but my money will only let me sit here and drink my beer while I think about it.

If this ends up needing a replacement transmission, what is compatibility like between K-bikes?  Can I use any K-bike transmission?  (i know the ratios differ between models).   But for example, a K75 transmission would work?  K1100?  Etc?

What do you guys think?

Sure sounds like it needs another transmission. I wouldn't even bother with finding out but I would say that a bearing has collapsed.  I wouldn't ride it as there is a possibility that it could lock up on you and the road rash just isn't worth it.
It may have been jumping out of 1st gear and that bends the selector and that makes it over engage in 2nd or permanently engaged a tiny bit so that the dogs get damages.
Make sure you get a transmission from a bike with a paralever which has the mounting point for the anchor arm below the drive shaft/swingarm.
There is no 6th gear.

Bah, sad day.  Time to pull the ol' BSA out of the garage and run it.

The 6th gear was my poor attempt at a joke   .

"Make sure you get a transmission from a bike with a paralever.."


this would essentially be:
16v k100's
k1's
k1100's

correct?

Yeah all the 16v bikes had the paralever rear end.
A 6th gear would be nice

Interestingly Maries last K75s ('91) had a paralever transmission casing though the bike never had a paralever.  The holes weren't drilled out though.

Al

You're correct, Al.  Some late model K75 transmissions had the tangs cast in the housing but not drilled.  Adding a paralever arm entailed drilling the tangs for the bolt (and fitting the rest of the paralever gear of course).

Most of the transmissions after 1989 had the mountings there, my 90 K100 had them and several late K75s I have worked on have them.

I don't now about your K's but mine has more than 6 gears...i use them everyday of the week.  Some are more help than others of course.

1st gear
Neutral
2nd gear
Neutral
3rd gear
Neutral
4th gear
Netural
5th gear


Mike

bwahaahahaha

I recon this guy has a sloppy foot rather than a fault in the gearbox ...gigles

 thanks for the laugh mike

Needs bigger boots  

The K1100 I have here has the smoothest gearbox I've tried. And I'm a tyre kicker, ridden a lot of them

I was on my iPad last night surfing for k100 thigns and watched a few videos on Youtube of people riding the things..sad i know!

What became pbvious to me is that the various clunks i get are normal as all the other riders were getting the same.

The most obvious is the big clunk from 1st into 2nd...on 4 seperate vids i heard exactly the same as i get.

And most of my netural episodes are casued by a gentle foot...i also ride a Yamaha and forget to engage a heavy left boot before i swap bikes.

I would get worried if i had gears jumping out but it's all good.

Mike

You can't have a light touch with the K gearbox. I have pulled one down at 300000km to replace bearings and seals and didn't know why I bothered it was still in top condition.
Transmission problems are rare.

Try an Rbike gearbox. 
The one here can't be changed unless you have shoes/boots on. 
It wore a hole on top of a pair of runners real quick. 
Very agricultural is the term if you want to be nice.

I know a guy in Lismore that has an R1150GS and it has done 4 gearboxes in 5 years (had to fix it 4 times) and still keeps the thing, don't know why.
He goes out with my brother riding and several times Peter has come to my place and got the trailer to rescue Paul.

I hear the 6 speed box is more of an issue. There is a design flaw on all RBikes, the clutch plate spline does not mate 100% with the gearbox spline. It's about 12mm short or 25%. I heard the 6 speed box makes it worse as people run in lower gears and the thump thump thump stuffs the splines quicker. Here's a pic from my RS, it's obvious after 153K Kms.



One would think they'd alter the clutch but it's a patterned part that suits many bikes. 
There is a guy in the states that sells modified clutch plates to fix it but ...

What a fuck up ...


But this is heading off topic  ....

Alright, well this is on the way.  It should be here by April at the latest:

http://www.ebay.com/itm/BMW-K1100-K100-K75-transmission-clutch-lever-pin-push-rod-BMW-K1100LT-K75-K100-/251481295109?pt=Motorcycles_Parts_Accessories&vxp=mtr&hash=item3a8d740905&nma=true&si=9Tf%252FMc%252Fs63stdDR2ZeCSqbcvU6c%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557


I'm thinking I'll likely pull the clutch out and replace the engine main seal / O-ring as I've been getting a little bit of engine oil out of that hole on the bottom of the front of the transmission.  

I really don't know how many miles are on the current clutch friction plate, but it doesn't slip.  I've noticed it takes a little longer to get it to grab all the way when letting the clutch in though.

Hey guys, update:

Transmission showed up, I should be good to go.  I just need to get the engine mail seal and O-ring, and maybe a clutch plate.  I can't seem to find the seal or O-ring in the online parts fiches.  Does anyone have part numbers for the seals?

Also, I think I know what you're all going to say, but is it necessary to replace the clutch friction plate since I'm already going to be in there?

O ring is part # 11 2 1 460 456

You will also have to replace the main lock nut and the clutch cover screws as they are one-time use items.

Lock nut # 11 21 1 460 673

Cover screws # 21 21 1 454 417

Output shaft seal # 11 11 7 666 186

As for the friction plate, I'd measure it and if it's not contaminated and within tolerance, re-fit it.

Agree with MikeP. If it had been a car I would say replace without question. But the Ks are kind to friction plates unless oiled upon. Always replace all visible seals weather car or bike, seals are cheap compaired to old seal leaking later and ruining work already done.

Check the pressure and drive plate for being dished, if the friction plate is worn to fit and not oiled then let the sleeping dog alone but if it is oiled then you may have to replace the whole lot.

Alright, so I went in there and everything looked fine.  It was a little wet on the bottom of the bell housing, but really nothing major.  So here's what I figured: I left the clutch alone entirely, didn't even take it out.  The whole clutch alignment process is pretty foreign to me and I don't wanna screw it up if I don't have to.  I would also have to wait about a week and a half for parts to come in, and I've got places to be!  So I just swapped transmissions and left it at that.  I figure I'll replace the clutch and the main seal / O-ring when the clutch starts slipping.

For now, though, the new transmission is smooth and the bike runs great!

Although, interestingly enough, the clutch sticks over night now.  Meaning that when I come back to it after a number of hours, the two plates are stuck together until I either rock the bike back and forth a little in first gear with the clutch lever pulled in, or put it in gear with the engine running and let it jerk forward for an instant.  It's not terrible, it just didn't do that before.

Bent pushrod?

Nah, pushrod movement was smooth when I installed it.  I think it's just that the bike was sitting for a week before I installed the new transmission.  Also, it's only when I go back to the bike after while.  After the grip breaks the first time, it's smooth sailing for the whole ride.

It's just the plates sticking together

I'll keep an eye on it and see if it needs up gradually going away.