I got my bike as a K75 Standard in October 1995 with 3600 miles on it. It was in pretty immaculate condition, but those few miles in 1.5 years wasn't much of a shakedown! With only a year and a half left on the warranty I figured I'd better get going, so I started riding it.... a lot. With those miles came a lot of modifications and accessories.
The original shock started leaking at 17,000 miles. Unbelievably (well, maybe not) BMW refused to cover this under warranty, saying that it was a wear item and the supplier only guaranteed them to BMW for a year. This is absurd; many owners have had shocks replaced under warranty with far more miles on them and it's not my concern what arrangement BMW has with its supplier. In all fairness, most of the blame probably lay with the dealer, Blackman's in Emmaus PA, who no longer carries BMW. I bought a Fox shock and have been happy with it.
Just before it went out of warranty, I had an oil leak at the clutch housing looked at by Bob's BMW and they replaced the oil pump, rear main seal (maybe clutch, I forget) as well as the driveshaft. I hadn't experienced any problems but I was happy to have the freshened parts.
One time the bike simply died, as if I had used the kill switch. I reseated the plug for the Jetronic computer and all was well again.
Occasionally I have experienced the slipping starter clutch. The starter drives a shaft which turns the alternator shaft, which is coupled to the engine by a one-way (Sprag) clutch. This tended to slip on the early bikes and was replaced with a different version, but they do occasionally still slip. It might happen a few times (several restart attempts usually results in eventual success) and then not again for a long time.
I did have a problem with driveshft splines wearing out over the course of 60k or 80k miles. I think the existing shaft and rear drive have been on for about 40k miles; they were replaced when the rear drive bearing failed during the summer of 1998.
On a camping trip I experienced a fuel leak that would run down the left side of the engine immediately after starting the bike, and would cease shortly thereafter. This was from a loose fuel line clamp (the crimp-type) on top of the pressure regulator. It was a simple fix but getting to it at night in a campground was a lot of work. I replaced that with a threaded clamp.
In early 2000 I had a problem where the bike would hardly run except at idle or wide-open throttle. Turned out to be a loose hose clamp causing an air leak between the air filter housing and the airbox.
Now, after 150k miles, there is a lot of oil seepage on the front and bottom of the engine. It's probably just the timing cover but it might be the head gasket.
The instruments, fuel pump, electronics, engine and Hall-effect sensor have never given me any problems at all. The speedometer is perfectly accurate, as far as I can tell.
In December 1997 I started some winter projects to upgrade the bike for more serious riding. The most substantial change was the addition of the RT fairing, and the electronics have been integrated better. Relevant pages are:
Here are a few of the accessories and things I've dealt with over the last two years. Fuel+, Sealed Battery, Headlight modulator, Autocom, and Valentine 1 (and a few others) are described on the Product Reviews page.
PIAA 910 series driving lights are really bright and throw a very useful pattern. I have owned lights which were too tightly focused, and this experience kept me from buying these for a long time. They are expensive ($220 to $280) but the beam pattern is just about perfect for all but the twistiest roads. Click here for a better picture of the fairing and lights.
The hand protectors (visible in the Standard picture) are great for keeping some wind off during the cold weather. Additionally, if the bike goes down they can prevent a lot of damage to the levers and heated grips. I keep them on in cold weather, since the wind hits the tips of my fingers.
The BMW luggage rack is great for strapping on stuff like sleeping bags, but the Reynolds Rack is bigger and has the backrest (shown).
Progressive springs were the first modification. As recieved, the forks bottomed badly under braking, to the point that the front end once bounced off the ground when I hit a seam in the pavement under full braking. The Progressives solved this problem completely.
Also required was a new rear shock, after my original unit blew out after 17K miles. I got a Fox Twin-Clicker which has worked out really well for me. At first I was nervous about adjusting it, but once I did, I got some real improvement over the stock shock.
On the unfaired K75, the BMW windshield is pretty good, and it looks better than any of the aftermarket ones IMO. The problem is that being fork-mounted, it is very sensitive to crosswinds. After a short trip on that bike with the windshield on and a return on a borrowed K75RT (frame-mounted fairing) I was sold on frame-mounted fairings which led to the conversion. On the RT, the electric windshield is without question the best windshield available for this bike. It may seem a novelty, but after using it in different conditions (often in the same day) you would understand why the versatility is so valuable. I tried a lot of different ones, though.
At first I used a trailer light adapter to activate the turn signals when I applied the brakes. It's just a Euro-to-American circuit box available at any auto parts store. It had the unfortunate side-effect of speeding up the signals when I turned and braked, but at least it attracted some attention and it was cheap. Better was the Priority Plus unit, which runs the turn signals at half power (using standard single filament bulbs) at all times and flashes them with the brakelight for a few seconds when you apply the brakes. Turn signals properly override the flashing.
Run'n'lites replace the turn signal bulb housings with dual-filament housings. The low filament burns all of the time. This helps give some apparent width to the bike when viewed from the front or rear in the dark. I use them on the front of the RT, but they are so close to the headlight that the effect is diminished.
Accordian fork boots are BMW parts which keep the bugs and grit away from the fork seals. The round-ribbed ones from the /5's are nicer-looking, but the newer style ones were the ones which fit best.
Heated grips are great inventions, but they don't fully do the job on an unfaired bike when it's really cold. Still, they let you wear thin gloves in much colder weather than would otherwise be possible. On the K75 Standard, the backs of my hands got really cold during the winter, because the mirrors deflected the air down onto them. Heated grips warm the inside of the hand wonderfully but don't affect the outside very much. Heated grips and heated gloves are the best winter combination, especially on an open bike.
Dunlop 491 tires last me about 20K on the fronts and 16K to 20K on the rears. After I wore out the original Metzelers, I never tried anything except the 491s. However, these are not very good on wet roads.
I use any kind of decent 20W50 engine oil, although it might be good to use a lower weight in winter. Normally I use Valvoline oil and a Purolator L10241 filter.
This bike is in semi-retirement at the moment, due to general deterioration from hard use and several accidents. This winter ('04-'05) I hope to restore the bodywork and title the replacement frame. I have since picked up another K75RT but will probably sell it once I get my original one back on the road.
Really, the cosmetic condition is most of what you'll want to look at.
If it has ABS, you should know that a new ABS unit (if it needs a new one) is $1000. Ridiculously expensive.
If you have the chance, pull the rear wheel and rear drive off and take a look at the splines. Follow the instructions on my web page for the easy way to do this. To just look at them, you might not even have to disconnect the speedo wire. It is possible for the splines to be bad at that mileage, and that will cost hundreds of dollars to fix (new shaft and then have Hansen's reweld and cut the splines on the rear drive).
You won't have a way to check the clutch splines. They will almost certainly be fine but due for a greasing based on time (if not already done during the bike's life). This is a DIY project that will take a day if you're not used to it. A few hours the second time.
You will probably hear the "K rattle" which is a gnashing noise as you accelerate. It cuts out abruptly at about 2700 RPM. Believe it or not, it's normal.
The suspension will be horrible if stock - par for the course. Don't let that dissuade you. It gets better once you spend some money on it.
See if the front brakes work smoothly. The stock rotors aren't very robust and it's easy to warp them. New ones (better ones, from Braking Systems) will be $350 or $400 total.
Make sure the radiator fan turns. These can seize after long periods of unuse. Look for oil seepage from under the oil/water pump and also from the rear of the engine; usually on the left side. Rebuilding the oil pump is not always successful; a new one is $250 or $300 I think. An oil leak at the rear is probably either a head gasket or a rear main seal.