I hope this diagnosis helps someone in the future, so I’ll post up my story.
Backstory: The bike is a ‘85 K100. I’ve had it for a couple years. Since I’ve had it, it would always start on the second attempt. The first attempt to crank, would always only crank for half a second then stop. Since it would fire up the second attempt, I just accepted it.
Fast forward to a month ago. I’m riding in to work one morning, and the engine dies on me. I try to restart it, and the starter only cranks for half a second, then stops for a second, then cranks for half a second, then stops, etc as I hold the starter button down. Can’t get it to start.
So at this point, I have a strange intermittent crank, and a no-run problem.
*remove/inspect and bench test starter. Operates fine. The power supplied to the starter is cutting in/out causing the weird starter activity.
*trace the power up to the starter relay. Gets a good constant power supply from battery. I check the power signal wire to the relay from the starter switch. It gets a good signal when I hold the starter button down.
Something is causing the relay to click on/off, so finally I test the ground. The ground is switching on and off. I use a jumper wire on that terminal of the relay and jump it to ground, and now get a solid crank. (Still won’t run though).
*I analyze the wiring diagram and see that the starter relay gets its ground from the Ignition Control Unit (ICU). Now we’re getting somewhere! Finally something that could cause both problems; no crank, and no run.
*I buy a used ICU and toss it in. No change...
Something else is telling the ICU to cut out.
*I look at the wiring diagrams again and also the flowchart Johnny suggested I check out. I see one of the only other imputs to the ICU is the Hall effect sensor.
http://www.motobrick.com/index.php?action=dlattach;topic=5067.0;attach=6480*I buy a new
Hall effect sensor, and install it. Cranks strong now!
*Reinstall the tank, and it starts up and runs!
Hopefully that helps someone. I’ve seen a few other threads with a similar on/off starter symptoms, but none got a difinitive solution.