My bike is a '92 K100RS which I have owned since new. It's built like a tank and has served me well over the two decades I've owned it.
In recent years, there have been some age related issues that have come up so I have finally gotten to turn some wrenches on it. Prior to that, I just rode the thing.
Interestingly, I've come across more people online recently that have been asking what to look for when buying one of these. I had written out a long email response for the third or fourth time when I realized it'd probably make more sense to just put something up here. For the vast majory of you who couldn't care less about my ancient oil burner, my apologies.
For those twisted few of you who like these bikes, read on.
I am familiar with the 4v K100RSA, but my understanding the K1100RS is essentially the same bike.
I am not a mechanic and this is not an exhaustive list, but based on my experience this is what I would look for if I were buying another one of these:
- Under the front of the motor, which will be difficult to see with the belly pan in place. there is a weep hole on the front of the motor under the oil/water pump. If there's oil or coolant leaking out, the pump needs to be rebuilt. If not, ask if the oil pump has ever been rebuilt.
(this one was leaking ever so slightly)
- Towards the rear of the motor where the transmission mates up, there is another similar looking weep hole. If oil is dripping out of that one the rear main seal or forward transmission seal need to be replaced. If it's bad, the clutch may have gotten oil on it, which may mean that it needs to be replaced.
- Ask if the rear main seal has been done.
- The famous spline problems that later BMW's have rarely plague these bikes unless they are very high mileage. However, it's good to ask, if the final drive and/or transmission input shaft splines were ever lubed.
- The clutch input splines can also be worn in high mileage examples. I have read, but do not have first hand knowledge, that symptoms of worn transmission input shaft splines/clutch input splines are hard shifting and frequent false neutrals.
- Oil around the gear shift lever is usually an indication that synthetic has been used in the transmission. I have been told not to use synthetic oil in the transmission or final drive. YMMV.
- Check for oil around the final drive boot near the rear wheel. That may be an indication of a final drive problem.
- Put the bike on the center stand, and grab the rear wheel at the 12 and 6 o'clock positions and see if you can get it to move laterally. Do the same at the 9 and 3. If there is enough free play to hear a click, the final drive probably needs to be rebuilt (expensive).
- With the bike on the centerstand see if you can get someone to push down or sit on the passenger seat to lift the front wheel off the ground. Move the handlebars side to side and see if there's any clicking or binding. If so the stearing head bearings need to be replaced. Mine tend to go every 25,000mi/40,000km or so.
- Again, with the front wheel lifted spin the front wheel and see if you hear the tell tale sound of warped rotors. Mine warped at about 65,000mi. My rear rotor warped in the first 10,000mi, but I so rarely use the rear brake now that my rear rotor looks new.
- Ask the owner the last time the front wheel bearings were done. Mine tend to go every 25,000mi/40,000km or so.
- Check around the fork tube seals and see if there is any evidence of oil. If so, the fork seals need to be replaced.
- Another common problem, hard to check for without some starting fluid or similar, is vacuum leaks around the throttle body boots.
- ABS faults that don't clear are also a common problem but are often just dirt on one of the sensors.
- Check for cracks around the exhaust system where the headers go into the catalytic converter. This is a known weak point in the earlier bikes (but I'm not sure if that issue was resolved on later models.)
(Even with additional bracing, my exhaust system failed completely at about 60,000mi. I now have a Remus stainless aftermarket exhaust system on mine.)
- Common complaints include that the bikes run hot. A hot running bike may be an indication of a cracked exhaust system which will tend to reduce back pressure and make the bike run leaner. Super high mileage is another symptom. (I got better than 60mpg before realizing that the exhaust had cracked.)
- The rear little stem mount on the frame that the seat pivots on when it lifts can break off.
That's all I can think of off the top of my head. I'll try to dig up some more photos and update the article with some more info. I you see any glaring errors or know of other things to look for, please let me know.
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