Hi, hope some body can help. I have a 1986 K100rt, the battery has 12.5volts but when I press the start button, there is only 7volts at the solenoid and bike won't start.
I tried following the wires back through the loom but no black & red wires come out on the switch side.

Welcome to the forum mate.
First thing is to be sure the battery is OK. Get it load tested as many batteries will show full charge and be fully charged but due to internal cracks and other things not deliver the current when called upon.
Next step is to measure voltage at the starter terminal, if that is low it could be the starter has problems.
HINT don't crank it for too long with a low battery as the starter relay will weld the contacts closed. (common BMW K problem)

WeKome to the forum Akarua.
Yep sounds like the battery. Just wondering where you are measuring the 7v from. K's don't have a solenoid but I'm imagining you're probing the terminal on the starter motor on the left??? The starter relay (sorta like a solendoid) is in the electrical box under the tank.

Tell us a bit about your K. How long you had it? Have you had it running?

Cheers

I'm measuring the power at the T fitting that goes on the solinoid if I put my screw driver across the main start terminals the motor cranks over.
The 7 volts from the start button seames to be the problem is there something reducing the flow of volts?
and thanks all for the input I'll get it yet.

This bike I've only had for a couple of years (she's ex police) but I had a K100RS for 17 years and had absolutely no problems with her
lots of great trips did the great ocean road from Brisbane down through Philip Island to the 12 Apostles and back up the inland road five and a half thousand K's very worth wile ride.

Arr right I misunderstood what you meant.
Try putting 12 volts and earth on the 2 terminals on the relay where the 2 pin plug is that will determine if the problem is in the relay or futher back in the start switch or the wiring. Also try pulling the clutch lever in as you start, if there is a change and it will start it means the problem is in the feed from the gauges.
The black/yellow wire also goes to terminal 4 of the ECU and terminal 6 of the ICU and the start button but does go direct to the start button so measure the resistance of that wire and check the 9 pin plug from the handlebar switch block to the harness for bad connection.
If you want a good colour circuit or manual with it then send a PM and I will send a link.
Keep going as you know just how good they are when they are working well.

Thanks guys I fixed the problem with a new earth to the solenoid works fine now.