I am new to the forum and just got my 1985 K100RS from my brother in North Carolina. Trailered it back to Michigan.  It hadn't been started in almost 2 years.  Siphoned the old gas, as much as I could, out of the tank and put in new. Put in a new battery and got it fired up.  At first it would run at idle and then quit.  The more I stated it and let it run, the longer it would run.  Took for a spin around my neighborhood, no issues.  Got out on the Road cruising at 45mph 2 miles from my house and she just quit.  Couldn't get it started.  Waited a few minutes and it fired back up. Got a 1/3 the way home and it did it again.  And then once more.  No idea what could be the issue. Though maybe old gas in the fuel system still? Plugged fuel filter? dirty injectors? ignition issue? I can hear the fuel pump running when I'm starting it.  Stumped.  Any help would be greatly appreciated! Thanks in advance.

I am going to guess that your ignition switch is dirty and needs to be taken down and cleaned up.  Seems to be a lot of that going around lately.

http://www.eilenberger.net/K75S/IgnitionSwitch/

Did mine last spring for the same reasons.

Thanks! Will try that!

Make sure you let us know how you made out.

I'd check the pressure on the fuel pump, new fuel filter. swap the plugs, check the air filter, inspect vacuum hoses, and check the small 4 small vacuum plugs. Best to eliminate the easy maintenance stuff first.

Another possibility is the Hall Effect Sensors on the front of the engine although I would expect the engine would need to run 10-15 minutes to get hot enough to have them break down and stop working.

While a fuel or vacuum problem is possible, they are different from electrical problems in that an electrical problem normally kills the engine abruptly.  Fuel problems usually manifest themselves as breaking up or loss of power which gets worse until the engine can no longer run.

Sounds like a fuel filter to me. I would be removing the lines from inside the tank along with filter and pump then clean the tank with that carb and throttle body cleaner. Spray it around inside the tank and give it a slosh around every 10 minutes for an hour or so then put 2-3 quarts of fuel in and do the same. hold the tank vertical wkile giving the lsat couple of sloshes to get all the crud to the back of the tank then syphon the fuel and crud out with a 3/8 clear plastic hose so you can see the crud coming out.
Check the rubber vibration damper around the pump and replace if needed (52mm damper ) http://www.euromotoelectrics.com/category-s/753.htm
Then replace all the bits along with new hose and I will say that the problem will most likely be solved.
If not then you have done something that needed doing anyway.

RicK G wrote: hold the tank vertical wkile giving the lsat couple of sloshes to get all the crud to the back of the tank then syphon the fuel and crud out with a 3/8 clear plastic hose so you can see the crud coming out.

Thanks for this tip on getting the last of the fuel and crud out. That's a big help!

Thanks for all the replies. I'm pretty certain it is electrical. it doesn't sputter like it is starving for fuel.  it just literally shuts down.  had it on a stand and revving at 3000 rpm and she shuts off like you turned off a switch. I inspected the Hall sensor to see if the wires looked bad. Saw a lot of discussion about this.  the look brand new. I will certainly go through all the steps suggested.  Needs to be done anyway. Will let you know how it turns out.

Certainly sounds like the ignition switch imo.
If the engine quits and there are no instrument lights like oil or battery, then it'll be the ignition switch as Point seven five suggested. Next time it happens try turning the key on and off till you get instrument lights.
If the engine quits and you still have instrument lights then it could be a failure anywhere from the hall effect sensors (HES) to the coils. Likely an earth connection. If it was HES, its unlikely both would fail at the same time so you'd still get firing on 2 cylinders at least rather than the whole shebang quitting at the same time.

Another possibility could be the plug to the ICU under the tank may be loose. We've seen that here a few times.

I know this sounds stupid but check the battery negative/earth terminal attachment and follow down to the main earth point on the gearbox. I had a similar problem and found out the battery terminal kept coming loose over bumps/high revving in low gear. Just cuts out dead, and sometimes when i turned ignition back on the dash lights would work until I hit the ignition switch then everything cutoff. A spring washer on the terminal under the earth cable has fixed it.

I put up with this for about a year...intermittently cutting out. Wriggled the ignition key and she would burst back into life. Eventually bit the bullet and pulled the switch apart, cleaning/burnishing the switch contacts, De-oxiting the whole thing.The switch is relatively simple to pull apart and re-assemble. The hardest part was physically removing the switch from the "dash-board". Been working great ever since.

Rickmeister.

RT, Thanks. I did check the earth connections as I saw that is some other posts as well.  Solid connection.  Took the ignition switch apart and it looked pretty good. Cleaned it anyway since I had it apart.  Put back together and fired it up.  Leaning towards fuel issue as previously suggested. Tank is perfectly clean on the inside. No sludge or residue.  When I changed the gas I put some BMW fuel injector cleaner in the tank. I've run it quite a bit, even at higher revs and all of a sudden the idle RPMs went up to where they should be and no delay in throttle response.  So possible restricted fuel flow.  Had been idling a little below normal. Still need to change the fuel filter in the tank, but I think I'm getting closer. Went for a short ride. No issues.... yet!

Don't be misled by a clean looking ignition switch, when I did mine it was almost spotless and yet after cleaning it I have had no problems in nearly six months. 

It looks to me like the main problem in those switches is that there is almost no pressure holding the contacts together.  With this marginal force it doesn't take much to interfere with a good connection.  Just another one of the handful of questionable design decisions they made with these bikes. 

Sounds like the injectors are getting cleaned out.  Johnny recommends doing an Italian tune up, putting a bottle of Chevron Techron fuel system cleaner in the tank and riding the piss out of the bike.  Open up that throttle and let the fuel flow and it will flush out the varnish that has collected during long periods of nonuse.  Two tanks cleared up a problem with rough highway speed running on my 16V RS.  The man knows what works.

Hey Point-Seven-Five, that is exactly what I used!  The BMW branded additive (in the states) is the TECHRON product.  Glad to hear I might be doing some things right. This bike is a little foreign to me as I had a Harley before. Like this one better though. Thanks!

Great!  You can get Techron at Advance Autoparts.  It will be cheaper than BMW stuff from a dealer.  Run a couple more bottles through before winter, and then lay her up with some Stabil.  See if you can find a source for corn free real gasoline.  That should clear up any fuel system problems caused by varnish and other crap.

Point-Seven-five wrote:Don't be misled by a clean looking ignition switch, when I did mine it was almost spotless and yet after cleaning it I have had no problems in nearly six months.

It looks to me like the main problem in those switches is that there is almost no pressure holding the contacts together.  With this marginal force it doesn't take much to interfere with a good connection.  Just another one of the handful of questionable design decisions they made with these bikes.
......

Agree. My initial inspection I thought they looked not too bad but what I found was it was a wonder they were working at all. All the contacts were covered in a hardened/glazed grease coating. I've read that this forms in the presence of arcing. You can see it in the photo below on both sides of the top contacts as well as the fixed contacts in the black base.
This is after a clean with a light coating of dielectric grease.

Make sure you have the 4 small springs in place under the contacts to presure them together.

Another thing I found is that the ballbearing/spring mechanism was not properly located and was why my switch was very slack and easy to turn. The switch located poorly at each position. Ensuring the ballbearing was located on the indent with the small spring greatly improved the notched positioning of the key .

Kaptian Hollister, found the same thing. Grease had gotten really thick and hard. Cleaned it all out and used some good Di-electric grease after cleaning the contacts. 

Point-Seven-Five, I have a filling station right down the street that sells ethanol free gas and I work with BMW so I can get the additive free!

Point-Seven-five wrote:Great!  You can get Techron at Advance Autoparts.  It will be cheaper than BMW stuff from a dealer.  Run a couple more bottles through before winter, and then lay her up with some Stabil.  See if you can find a source for corn free real gasoline.  That should clear up any fuel system problems caused by varnish and other crap.

BMW's specified cleaner is Chevron's Techron, as most folks know, but their proprietary formula has 50% more of the additives that clean than the off-the-shelf Techron from an auto parts store. You pay more - but not much more - 'cause there's more of the goods in it. It's now available in a small, easy to carry about bottle for about $7 and in a larger, garage size for about $13. The 5oz is 07 51 2 412 378 & the 20oz is 82 14 0 413 341.

For regular use in every tank, and when ethanol-free petrol isn't readily available, I use an ounce of Star-Tron ethanol eater/stabiliser which also treats stored 'gasoline' for up to two years.

Back to square 1.  Bike quit and won't restart. Just sputters a bit and doesn't start. Replaced the fuel filter, cleaned the tank and fuel pump screen, new spark plugs and nothing.  Have good spark on the new plugs.  Fuel pumping to the injectors. Even pulled the new spark plugs and they have fuel on them but nothing.  Vacuum hose to the pressure regulator by the throttle body is somewhat hard and doesn't seem that tight.  Found this out when I was depressurizing the fuel rail. But thought that would just cause some erratic running issues, not kill the whole thing. It ran great for about 2 hours then died.  What am I missing?

Check the electrical connections to the fuel tank. Yours being an 85 the connector will probably be at the front left. Check for corosion and make sure the pins are connecting with the sockets, they get a bit lose and then are eratic. The pins may need to be opened up a tiny bit to get a good connection.
Did you clean the ignition switch like Captain Hollister suggested, they can have you running around in small circles and are a continuing problem with the later airheads and Ks.

Sounds like a degrading issue. Ignition sw. can continually get worse till it just won't work.... but if you're getting fuel and spark, it may be something else.

Check your ICU plug is plugged in firmly. They can become progressively loose. Under the tank behind the headstock. Check your main earth connection while you're there.

Might be worth checking the impedance on the water temp sensor. FICU plug pin#10 to earth should read 2.5ohms at 20°c. It may not be earthing properly which means it will tend to flood.
Read this... about ¾ way down
EFI/Electronic ignition troubleshooting

Could be a battery/charging issue. You bought a new battery but what amp/hour rating is it. It may not have been charging properly due to a bad alternator.
Measure the static voltage across the battery. Should be about 12.8V or more.
When you get the engine running, charging voltage should be around 14V above 2k rpm

Thanks guys. Rick G., Yes I did clean the ignition switch. It was one of the first things I did. I will certainly check out the other things suggested.  I also contacted a guy I know at Motorrad at BMW NA HQ to see if they have any insights as well. Will keep plugging away! A this point I will have all winter to figure it out.