Hi everyone!
   My wife gave me a 1985 K100rs for my birthday. I've spent the last three weeks getting it running. It had sat for about a year before I got it.
   Now it's running, so I'll be riding it. I want to make sure that I don't hurt it or leave myself stranded by missing something obvious. I have a background in restoring antique vehicles, so I'm used to dealing with mechanical issues, but this bike is unfamiliar to me and much more complex than I'm used to. It's 43 years younger than my next newest vehicle!
   I've always followed what I think of as the 5, 50, 500 rule: Don't ride more than five miles at a time until you're really sure the bike can do it completely reliably. Then don't ride more than 50 miles until the bike is trustworthy, etc.
   So here's what I'm wondering: What are the things I should be checking for or paying attention to as I take my K through these milestones and learn to trust it for long distances? I've already done the pre-ride inspection things. Fluids, switches, lights, tires, etc. I have the service schedule in the Clymer book. So I have that stuff covered.
   But what's the inside knowledge from riders? What are the things where you say, "Watch out for X, or your Y will explode."? Is there anything specific I should be paying attention to at those various mileages? Things that are fine for five miles, but will give me trouble on fifty mile rides?
   I'm particularly interested in things that are specific to the Ks, or specific to a more modern vehicle than I'm used to. I can (and have) diagnose and repair a clogged carb jet by the side of the road. But injectors? I don't have a clue.
   Thanks in advance for your advice!

Have you done the service on the splines and checked the seal and O-ring on the output shaft.
With a bike that has been sitting for some time the oil seals get hard and will start to leak after a few thousand km. The main seal behind the clutch can be picked up on with oil comming from the hole under the intermediate housing just forward of the gearbox. Eventually the oil gets to the clutch. The O-ring is on the main shaft behind the nut for the clutch carrier and gets hard and leaks from the same hole, both can be done at the same time. The rear drive spline gets dry and rusty then wears out very quickly and the input shaft to the gearbox also gets dry and causes difficult down shifting and will wear so bad that you lose drive.
Oil/water pump on the front of the engine can suffer from the same fate with hard seals. There is a weep hole right at the front of the sump (oil pan) and oil or coolant or both will escape from it but the hole does get blocked easily so keep an eye on the coolant in the expansion tank for oil contamination.
The pump is an easy fix with a kit for around $40 providing the pump shaft is not rusted on the seal area.
They are the things that will show up within a few thousand km.

Mechanically, the Brick engine is just that, a brick.  Most problems are electrical in nature and usually involve a bad connector or switch/relay contacts.  Deoxit is your friend.  Shoot a little into every electrical connecting point you see whenever you are working on your bike.  Pay particular attention to the connectors on the engine computers.  They have been known to cause problems if not properly engaged.

If you haven't done it yet, I would suggest cleaning the ignition switch.  Seems like there has been a rash of problems with them lately, including one of my bikes last spring.  Dirty contacts in that switch will cause some bizarre intermittent problems that will make you crazy.

The Hall Effect Sensors(HES) that control the ignition can cause problems, but only when hot, requiring extended riding on a warm day.  They will cause the engine to quit when hot, and then run again after a half hour or so of cooling down.

Most of the problems with rubber bits will show themselves at idle.  If she idles nicely, you can be reasonable sure they're ok. 

Last, the sprag clutch on the starter can give you problems by not engaging.  I had a problem with it on my RS this Spring and cleared it up by doing an engine flush.  I drained the oil and filter, and replaced it with 3 quarts of cheap SAE40, a can of Seafoam and a new filter.  I rode the bike easily for about a half hour(20 miles) and did an oil change using a new filter and Shell Rotella T6 full synthetic that I got at Walmart for $22.  Clutch has worked perfectly since.   

Basically, these bikes are made to be ridden.  Gas it up and get it out.  The more you ride it the better she'll run.

Great! Thanks for the advice! I think my biggest hurdle might be my own mental block. I've literally never owned a reliable vehicle. I'm used to about a 2:1 ride time to service time ratio. Sounds like, after some precautionary work, I just need to relax and enjoy the ride. I think I can manage that!

I got my first K in 2010, a 92 K100LT

At the time a BMW was not even on my shopping list but a test ride convinced me.

It needed a few things so got them all done, some were on the hit list and all in cost me under €400 which covered the cost of sorting brakes and master cylinder seals kits, clutch cable, mirror, timing chain check, full service, sundry minor jobs. After that it performed reliably apart from a battery failure which was sorted easily.

I had one issue which led to me finding this forum and it was the best thing ever, for advice and know how and some other members offering to swap parts etc.

In 2013 I got a 1984 K100RT.

Rode it home and took it apart on the patio on a sunny weekend and went at everything. Full service, filters etc etc, checked and cleaned electrical connections, switches, painted part of the frame, went through it with a fine comb. Since then it has performed as it should and with help of the forum you just get to know the easy fixes and the things that go wrong. Its really worth reading your way through the forum because a lot of quite complex problems have some amazingly simple solutions. Advice on things like ignition can also be applied to the gas tank cap [check out Sidecar Paul's hole solution to a stuck gas tank cap] and many more. If you take the time to do the preventive stuff the K is certainly as reliable as any new machine. If you know the forum well then when you do get a problem at least the solution will be to hand.

I enjoy sailing where the work to use time ratio is about 10:1 but I can honestly say that once you do the precautionary work the maintenance time on the K is minimal provided you attend to the little things.

I know exactly what you mean Jon!, it was the same for me when I got a much more modern car!, my old car was, well, a bit maintenance intensive at times, never stranded me anywhere, old and simple meant it was fairly easy to get going again!.

When I got a much more modern car, the fact that is was so complex made me feel very uneasy with it, but I gave it a good service, checked it over as much as I could, and just drove it!!, has been very reliable.

Same thing with most of my old motorcycles, barring my faithful R65, now I have a K75, its a lot more complex, and is in bits at the moment, that dreaded engine clatter!...

But once back on the road again, I think its simply a case of making sure all the maintenance is up to date, check over what we can, and just ride!.

Oh, and some breakdown insurance, just for peace of mind.....

I do agree with a few shorter trips, until one gets to know the machine, then just enjoy!,

John

The ignition switch and the fuel cap lock are 2 things that can strand you away from home but are easy to maintain ensuring you don't get caught out.

The ignition switch will give you an intermittant electrical failure which will lead to complete failure eventually. Its relatively easy to disassemble, clean and regrease with dielectric grease.... all good then for another 50k. Links to instructions are on the forum or just ask.

The fuel cap has caught a few. Once stuck you will need to drill a hole in the top of the cap to operate the catch (a work-around by sidecarpaul). Before this happens, pull the cap apart. A clean and regrease will keep this lock working properly. There is also a gasket kit for the fuel cap. Make sure the drain hole to the left side is clear. It drains to the r/h rear spigot to a collection cup and then rubber tube to the rear. If the cap drain hole is blocked, water will get into the fuel.

There are probably a few spares you need to carry.
Clutch and throttle cables.
A spare SP or two.
I carry a collection of tools but never really needed to use them.
A puncture repair kit which I have had to use.
I also carry a spare starter relay and a set of hall effect sensors if I'm on a long trip

Basically, as the guys have said above, keep the maintenance up to date and just get on her and ride/enjoy.