Duplicate post from k-bikes.com...

I have an '88 K100LT that loves to kick itself out of second gear into neutral. I believe this is a relatively common problem, but the odometer only has 30k miles which seems early for it to be happening.

As I understand it, these are the available options:

1. Rebuild gearbox
   
2. Acquire a replacement K100 gearbox
   
3. Acquire a replacement gearbox from a paralever bike and go through that upgrade at the same time
   

Having trouble deciding which one to do because I don't know the costs of each, I don't know where to source a confirmed-good used transmission, and I don't know who does K-bike transmission work in the Los Angeles area. Any advice from folks who have gone through this before? Thanks!

undo the front cover on the gearbox when out and examine the cam assembly on the selector drum ...if rotating around the shaft too much will not select gear correctly , and hold in gear that is ....but usually from racing changes sheering off the second gear dogs leading edge , and now a bent selector fork .


just a heads up

good luck

Oddly enough, I spent half of yesterday dealing with this problem on the K75 Madass, that's a 30/40K gearbox and it has suffered all the inexcusable abuse that Charlie describes.
It got very bad very quickly - once it starts jumping in and out of gear the internal wear increases and trying to hold it in second with your foot on the gear lever (and persisting with the racing gearchanges aaargh) only leads to a quick demise.
Anyway, the second gear selector fork is toast - worn to about half thickness! The selector drum is mashed too.
I've had the same problem before (on my K100) and swapped forks, but tried to get away with reusing the drum, which didn't seem too bad. That fix didn't work for long though.
This time I'm lucky enough to have a good set of internals that I've swapped in, the drum and forks have no noticeable wear (they have a hard coating which is completely intact).

What I did notice this time though (and the reason for this post) is that the detent disc (the 'star' on the front of the selector drum) seems to be pressed in place and prevented from rotating by a small roll pin. On the worn drum there is a load of slop, maybe five or more degrees of rotational play, whereas on the good drum, there is still some slop, but much less.
The detent disc is what holds the whole thing in gear, so a sloppy disc is not going to help matters - maybe it is even the cause of the whole problem, since the cam groove on the drum is very pointy for second gear, and slight slop (and a bit of wear too) will let the follower/fork fall off the point in the cam groove.

Maybe detent disc slop is the root of the second gear problem? Anyone else got any thoughts on this, or a fix for the slop? (I'm still figuring one out)

I'll post some pics after round two later today.
Cheers folks

Sorry for the slight thread hijack btw.

Edit
Dang, I've just re-read Charlie's post and realised that the 'cam' you've referred to is what I'm calling the 'detent disc' (In my parlance the 'cam' is the drum itself) - so you're on about the same problem as me?

Here's the second gear fork


And the selector drum - the worn away point is where the fork follower is supposed to sit, this is what's supposed to hold it in second gear


This is where the problem might start - the drum on the left is the 'good' drum that I used as a replacement. But you can see the roll pin has already crushed, and this allows the detent plate to slop. You can see the detent plate and the notch in it, which locates on the pin, in the second picture below.



The above picture is the 'bad' drum. It didn't have a roll pin, but instead a solid pin which was a sloppy fit in the worn hole - the pin is on the lid in the picture. I decided to dig out my dear old departed dad's taper pins and reamers for a fix. He was a machine tool fitter by trade and these, amongst other stuff, were the tools of his trade for fixes like this. He taught me. That's his fancy handhandwriting on the label; he labelled everything. He must have known dementia was lurking somewhere in the future.
I've already fitted a taper pin to the bad drum in the above picture, as a trial. It seemed to work, so..


The detent plate is now a snug drive on fit over the slightly oversize pin.


Nice one dad x

instead of using a roll pin , I actually drilled the selector drum to a depth of about 8mm instead of the usual 3 mm   with a 1/8 inch (3.22mm)drill (next oversize I could figure ) and made a new pin to fit  about 10 mm long ...with a section 3mm to fit the keyway in the cam
I figured that the extra length through the selector drum metal should hold up for a reasonably long time , and removed almost all slop in the cam movement completely .

just the way I thought things through that made sense to me , at the time

Good thinking Charlie.