I think my '85 K100 had a couple overlapping problems. I seem to have fixed it stalling and not starting again by replacing the Hall Effect Sensors (also replaced air intakes and all fuel lines and and cleaned all electrical grounds).

Now however it still will run for about 15-30 minutes, then starts stalling and won't start. If I wait a few minutes it will start up again. It seems it's worse when I'm going slow, if I can get it up to 20 or 30 MPH it seems to be OK (that got me home). A couple times the oil and battery light lit when it stalled.

Things I haven't done
- no insulation under gas tank (will add)
- coolant is old but not disgusting (will replace)
- didn't replace vacuum hose
- engine breather hose doesn't go into air intake - previous owner blocked the entry to the air intake and the end of the hose rests on the engine

What should I consider troubleshooting next?

- ignition coils? (I haven't checked to see if they are the original kind that were bad)
- could a dirty radiator cause too much heat that would cause this? (Or would I see the heat warning light?)
- there's no plastic tray under the gas tank, and I understand there should be - would that make wires heat too much?

Oil and Alt light coming on is normal.
When you start it cold then let it warm up does it idle OK without the choke lever being opened.
To me it sounds like it could be air leaks in the intake system between the engine and air filter. As the Ks age they are very prone to these problems as the rubber components tend to get very hard and crack.

It needs the choke lever to start, but then runs nicely when cold

I just discovered i forgot to replace the front airbox clip - though it's still pretty tight with the filter (using a K&N filter the previous owner put on). Replacing the clip now.

I did replace all the rubber on the intakes so that shouldn't be leaking there

When it quits, do you lose the spark on the plugs?  All four or just two?  As I understand it, the older K100 engines are known to have ignition coil failures.  I can't recall what the failure mode is for them, but there was a design change in the late 80's that is supposed to correct the problems the early ones had.  Perhaps someone with more knoowledge can suggest a test for them.

Normally, the only parts of an engine that stop working when they get hot are the electrical components in the ignition circuit.  You have replaced the weakest point which is the Hall Sensors which leaves the coils and the ignition module under the tank.  You might want to insulate the area between the engine and the tank.  the fact that moving helps means air flow under the tank is helping cool things.

I may be wrong, but I believe the new coils have dark orange terminals where the early ones were all black.  Again I am working from what I recall reading as I have no personal experience with the early coils.

timz10000 wrote:Things I haven't done
- engine breather hose doesn't go into air intake - previous owner blocked the entry to the air intake and the end of the hose rests on the engine

If you are referring to the part indicated by the red arrow in the attached illustration from the MAX BMW parts fiche, replace it and reconnect it. That may help.

The early coils were a very dark grey to black and were very prone to the secondary coil going open circuit which eventually led to total break down of the internal insulation between layers of the coil windings and little to no spark.
It was around the release of the 87 model that they were changed to the brown with orange towers on them, a much better coil and a straight replacement.

I wonder have you looked at valve - shim clearances ...

if too tight could well be an issue ...I have a memory that the exhaust valve clearances do regress over time ...and worth testing = measuring before you do ANYTHING else

it isnt hard ...but needs to be done before any symptoms  could be diagnosed ..I guess your looking for 10 -12 thou clearances cold

some one will correct me if I got the clearances wrong ...but 6-8 thou is likely too tight

good luck

Mine were way tight a while back on the old girl and she was stalling at the most inopportune times. However, she would start back up straight away. As you say, worth checking asap as too tight can lead to burnt valves and I would be checking those just to be sure.

A guy I know up the coast struggled with the exact same problem described above for a long time. 20 mins down the road the engine would stall. I suggested it could be the temp sensor. He checked it and sure enough it failed at operating temp. He bought a new one and after he installed it.... same problem!! I couldn't convince him that the new one might be dud as well but after a few days he put the meter on it and sure enough it was failing at operating temp.... go figure. Anyway he ordered another temp sensor and this time problem fixed.
DMM on FICU pins #10 and #13 (count from the lead end) should read 2.5kΩ at 20ºc and below 300Ω at operating temp. Detailed instructions are listed on the Trouble Shooting page off the Portal tab.