I took the K100RS for a spin today (beautiful weather in Canberra for once) and found that the girl just does not seem to ride right.   
When I picked the bike up I discovered the right hand fork had slid through the top triple by about 20mm.  I fixed that and then checked all of the bearing, etc which all seemed OK.
Today on the ride the bike seemed to be very very harsh over mild ripples and had a mind of her own over seams in the tarmac.  I stopped to check the Koni rear shock and set it back to 1 (of 4) damping settings.   This seemed to settle things down a little but it was still harsher than I remember.

I bought her home and drained the fork oil.  Right side (the one that had slid up) had 400ml of what looked like liquid titanium (almost silver in colour).  The left drained 360ml or worn oil more black than clear.

I refilled the forks with 360ml of 10W oil and gave the front end a few bounces to settle everything before putting the carps back on.

I have to say that I am not sure that it made any difference.  If I hold the front brake the forks compress a good 4-5 inches and rebound PDQ before settling down after one 'bounce'
I am a life long Ducati rider and the way the forks worked would worry me a lot... but is that normal for a 1991 K100RS ?

According to the info I have you should have 380ml in each leg and the 8V RS was the one with 360ml capacity and that extra 20ml can make a big difference.
Also from what you say you have the KONI with the red adjuster at the bottom and they were renouned for a stiff ride. I have one on the K1100LT and it is a bit harsh till I load it up for touring then it is good.

Dammit, I have done it again (Koni v Ikon) - The bike has a reasonably new Ikon shock with the adjuster at the top.  
The bike also has Ikon progressive springs in the front - does that affect the amount of oil ?

It shouldnt make any difference to the quantity of oil but the best people to ask would be IKON.
I would be interested to hear how the IKON goes I have heard a very mixed bag with regards to performance with them.

Rick,

The VIN on the bike decodes to a 7/89 build even though the ADR lists it as a '91.   Do you think 360 or 380 ml ?

I can add the additional 10W if you suggest, but if it is still wallowing, would you suggest going heavier ?

The difference came with the introduction of the 16v engine and paralever rear drive and 305mm front discs
If it has the paralever and 16v then 380ml if its earlier then it's 360ml
The para lever was introduced on the production line about July/August 1989 but none made it to Australia before about May 1990 and many didn't get plated till late 1990/early 1991.
If you still aren't sure give me your engine Number and VIN and it will take about 1 minute.

can I add .....mind of its own over seams in the road is a pretty common issue to do with tyres ...and wear on them etc ...including age

what do you have ? age ? guesstamate ?  tyre pressure ?

just a heads up oz ...

A short story of the bike is:
I sold here in 2009 and found her for sale again a couple of weeks ago - so I bought her back home.

She has the original Lazertec (?) on the front that I installed and has what looks like a reasonably new M88 Marathon on the back.... I can't be sure and I aint going out to the shed as it is bloody freezing here ATM LOL

The rear looks more like a car tyre than any bike tyre I have seen recently and it seems to be a really hard compound. It still carries the little beardy things on the outer edges so it can't have had a lot of use.
I am running 32F and 36R

Tyres have the manufacture week/year embossed on them. 

Mine had an 09 front that had 90% tread when I got it and the bike wallowed in bends and tended to pull into slow corners. I put a cheap new dunlop on the front and all was fixed. Went for a run today and the back was skipping out on mountain roads. That tyre still has the beading around the edge and 100% tread but it's dated 2010 so I guess it also needs to go

I'm with Charlie re tyres especially now you have said what's on the bike . Dump them and put on your choice of a few recommended Brands. Search tyres here or go straight to Pirelli Sport Demons if you don't want to muck around. +1 size from stock, I think it's 140 for the rear on a K100? someone will correct me, +10mm anyway.
These may not be the absolute best but will give you a benchmark for ALL other handling issues. Time and time again there are guys reporting outstanding improvement with new rubber. Myself incl. Matched pair is my pref but others are fine with diff brands. Also your pressures should be higher with these newer styles, high 30s front low 40s rear, from memory ,someone will chip in see mine is 75 so a bit different sizes etc.. $450 + hit but.....?

Thank you all gentlemen.
This morning I got up to have a really close look at the tyre, suspension, etc.

I had filled the forks with 360ml of 10W which seemed to make the front end a little less skittish when I went for a short ride around some roundabouts.

Sitting down and having a close look at the tyre would suggest the rear was manufactured in the 29th week of '08 - so that make it about 5 years old.

The front seems to show 2 numbers so it is either 34th week of '10 or 16 week of '08

So a new pair of tyres will be fitted soon.

Rick,
You got me thinking about the rear end and Ikon shock.  I took it off this morning and when I turned it around it would appear that the shock might have blown at some point.  There was evidence of old oil that had run down the inside of the shock.   I quietly went out to OZMaz project bike and 'liberated' the shock off her bike.

A quick lap around the block was enough to tell me that the 'newer' Ikon shock was indeed buggered.

Well dont buy another IKON as they do have some serious quality problems in particular the last 3 years.

The shock I would recommend is the RAM SHOX  http://www.realmengineering.com/page15.html

I have heard nothing but good reports on them from all over the planet.

I fitted one to my K100 just before it was collected by the new owner and after a short ride was very impressed and also fitted one to a K75S that I did work on recently and was once again very impressed at the road holding and smooth ride.

One of the guys on the motobrick forum had an IKON litterally fall apart while riding and a friend here with a K100 had a similar experience while lifting onto the side stand. My brother has a R80GSPD and fitted an IKON  and the spring just lost all tension and was replaced 3 times until he got a new spring made localy but now I have heard it is leaking around the shaft after only 18 months.
The local bike shop won't sell them any more because of too many warranty claims.
It just goes on and on the list of problems.

You may recall I was having trouble with an evil handling bike and swapped the Ikon shock with a standard shock from a 1985 K100.
I have no idea if the OEM shock is the original or if has been replaced at some time but I took the bike for a quick pedal along Cotter Road after the Veteran and Vintage Gluewein (sp?) event at the weekend.

The bike is now much more civilised with the OEM shock.  It is a much better match to the front end and I was able to push it much harder than I would have dared a couple of weeks ago.
I emailed the Ikon people in Albury about the problem and they suggested it would about $100 to rebuild the shock.

Now the exam question is - do I have the Ikon rebuilt or do I invest a little more money in one of the other products people have suggested.

Thoughts please.

OZHoppo

Seeing as you asked, throw the thing in the garbage and write it off to experience and get a RAM
http://www.realmengineering.com/page15.html