K100RS 1988 2 valve non ABS - Running rough

[Cafe racer]

  HELP returning from a nice run out the K stated to run rough, but got me home.
History
New fuel pump, mounting and filter etc.
New fuel pump relay
Fully serviced oil wise...

so far I have done...

I thought maybe a blocked injector, was like it was running on 3 cylinders now and again.
I have bench cleaned the injectors, all were operating and all performed about the same.
All 4 spark plus were the same colour and looked normal as can be.
I tried it with a full tank of fuel, no change.

The only thing I have noticed is a little more than normal play in the twist grip (I cant see how this could cause the running rough system)

[frankenduck]

Things I would check:

1 - Spark plug leads:
a) make sure that all four are well-seated on the spark plugs
b) measure the resistance of each spark plug lead - they should all roughly be the same

2- Ignition coils - measure the resistances across the coil terminals like so:


Pictures are from Bertrand Vogel's piece on K100 troubleshooting that can be found at this link: https://bmwmotorcycletech.info/K100%20troubleshooting%20Starting.pdf

[Laitch]

After completing frankenduck's assignment,  check the condition of the z-hose connection from the crankcase to the air box. You could pull it out to check for cracks or you could idle the engine and spray propane on it. If the idle increases, it's leaking. You could also spray carb cleaner on it but that is more likely to ignite if it pools then gets sparked. A replacement z-hose costs around 10USD. Check all the throttle body vacuum ports. Three should have black caps covering them; one should have a hose attached to it. The caps should not be cracked. Disconnect the hose; fuel should not drip from it. After that, take it for a ride. The hose is circled in the photo below. It's on the shifter-side of the moto. The green arrow points toward the coils; the orange arrow points to the throttle switch.

Next:
Inspect, clean and tighten the battery connections and the battery's ground cable connection at the transmission. Clean the fuel pump's four-pin plug connection under the fuel tank's right side. Clean the frame ground connection beneath the tank, all the terminals connected to it then be certain it's tight. While the tank is up or off, clean the ignition switch plug.

[Chaos]

Another somewhat unlikely scenario, but it happened to me, is water accumulating in the tank enough that the pump can sporadically pick it up.  Can of HEET cured it.

[Cafe racer]

frankenduck and Laitch
Thanks for the suggestions.
Coil resistance is 3.5 and 11.8 on both coils (not same as the image posted, is this an issue?)
z-hose looks to be in good order.

This weekend I will remove the tank, and carry on with the checks.

[Laitch]

Coil resistance is 3.5 and 11.8 on both coils (not same as the image posted, is this an issue?)
z-hose looks to be in good order.

3.5Ω resistance on the primary is approximately 60% greater resistance than the Testing for K Models manual specifies, so that seems significant to me. The manual doesn't specify an allowance of deviation; it only indicates 2.2Ω as the spec. Maybe others will chime in with their numbers.

Did you propane test that z-hose? That's a more accurate way to verify its condition.

[rbm]

...  A replacement z-hose costs around 10USD.

Really?  Wow!  It's $35.00 Canuck bucks up here north of the border.  Ripoff.

frankenduck and Laitch
Thanks for the suggestions.
Coil resistance is 3.5 and 11.8 on both coils (not same as the image posted, is this an issue?)
z-hose looks to be in good order.

This weekend I will remove the tank, and carry on with the checks.

Resistance on the primary is a bit too high.  Should be very close to 2.2 Ohms.  Are you confident in the accuracy of your Ohmmeter?  It could need a fresh battery.

Also, remove and visually inspect the Z-hose under a magnifier.  Cracks can be invisible to the naked eye and hidden unless the tube is flexed.  That's hard to do while still on the bike. I changed a Z-tube on a friend's K75.  It looked ok on the bike but literally disintegrated in my hand when I removed it.  Totally rotted out.

[Laitch]

Really?  Wow!  It's $35.00 Canuck bucks up here north of the border.  Ripoff.

Good news, rbm! You can curb your envy. It's been a while since I bought one. The upper one is from Euro Motoelectrics; the lower is from MAX BMW.