I formerly posted at this link but thought I'd start a new thread for clarity

My 85 K100 starts running rough after about 10 minutes. I'm wondering what to try next - any suggestions welcome! 

Symptoms: It starts and runs pretty well when cold. After say 10 mins it doesn't cut out completely, but starts to sputter and run really rough. The battery light comes on at times when it's about to die. I can still rev it high and it seems to run better at high rev (but still not great)

When I cut the motor and leave it for not too long (guessing 10 minutes) it'll start up fine again.

I have a parts bike so I can try swapping things (parts bike hasn't run in many years, but was out of the weather - tank not functional tho)

What it's not:
swapped coils
swapped ECU
swapped ignition control unit
fuel lines replaced inside and outside tank
fuel system has pressure / fuel pump working
thermostat in radiator replaced (failed hot water test so I got a working one)
thermostat temp. sensor replaced

What it might be:
I replaced the Hall Sensors themselves but didn't replace the wiring
When tank is low on gas and I open it, vapor comes out - possible vapor lock? (When I open cap I don't get whooshing of air)

Other guesses:
could fuel pressure regulator or vacuum hoses cause this?

Sounds like you are getting a vacuum lock in the tank. Try running until it starts to play up and with a spare key pop the fuel cap. If it starts to behave it's self you have found the problem. The cap can be pulled apart and cleaned and the cap service kit is still available.
Regards Martin.

MartinW wrote:Sounds like you are getting a vacuum lock in the tank. Try running until it starts to play up and with a spare key pop the fuel cap. If it starts to behave it's self you have found the problem. The cap can be pulled apart and cleaned and the cap service kit is still available.
Regards Martin.

Cool thanks will try that tomorrow.

timz10000 wrote:

MartinW wrote:Sounds like you are getting a vacuum lock in the tank. Try running until it starts to play up and with a spare key pop the fuel cap. If it starts to behave it's self you have found the problem. The cap can be pulled apart and cleaned and the cap service kit is still available.
Regards Martin.

Cool thanks will try that tomorrow.

I tried it but that wasn't the problem. (Ran for 20 mins til it died, wouldn't start. Waited 15 mins, restarted with the cap open, ran for a few mins then cut out again and wouldn't start).

Any recommendations about what to try next?

When it cuts out pull the spark plugs and report whether they are wet or dry.
Regards Martin.

When you say replaced the Hall Sensors, was that new ones or replacement used ones?.......

Funny I started thinking of Fuel pressure regulator too and see you mentioned it.

Battery light glowing on at very low revs is not unusual.

Wonder could that fuel tank connector plug be acting up and causing loss of fuel pressure...? Wiring to that goes alongside and near the radiator pipes on the bolt down early 85 tanks.

92KK 84WW Olaf wrote:When you say replaced the Hall Sensors, was that new ones or replacement used ones?.......

Funny I started thinking of Fuel pressure regulator too and see you mentioned it.

Battery light glowing on at very low revs is not unusual.

Wonder could that fuel tank connector plug be acting up and causing loss of fuel pressure...? Wiring to that goes alongside and near the radiator pipes on the bolt down early 85 tanks.

With the Hall Sensors I bought new actual sensors, and put them on the plate and wired them to the existing wires (but it seems possible the old wires get hot and maybe fail? I'm thinking I'll try putting in the Hall sensors from my parts bike since the behavior sounds like it could be that.)

I'll try swapping the Fuel pressure regulator. 

With the fuel tank connector plug - the mechanic I took it to said the fuel pressure was good when the bike was cutting out (He thought the problem was probably the ECU or the ignition control unit, but then I tried those and neither fixed it).

I would be taking a new look at the hall sensors because what you describe is very typical of them.