I'm going to confess here, I can build a Norton Racing bike that will win national championships, but a little BSA C15s has me absolutely stumped. We took on a friend's bike to get roadworthy for him as a favor. Wow... this has led me down a dark wormhole of the unknown and countless hours of wasted time on an esoteric electrical system.
In short - this bike has a 4-wire alternator in the cases (BW/R/NU/N). I have a C15s wiring diagram and have confirmed and traced all the wiring, insuring it is as it should be. The bike runs well, but only when the lights are in circuit. If I disconnect the lights the bike dies. It seems that the taillight is completing the ground somehow. So I started eliminating connections to trace the problem. When I disconnect all the wires from the alternator except for the ignition power (BW), and the Ground (R), the bike won't run. When I connect the headlight/taillight power (NU), the bike runs (but the lights don't illuminate, even though they're in circuit now). It's as though the powered lighting system is creating a ground loop from the alternator to make it function. As a test I connected the ignition power as it should be (BW) to the coil and points, then connected lighting power output wire from the alternator (NU) directly to the red grounds on the bike, and it ran perfectly. So perhaps there is a problem with the red wire from the alternator? Or a problem somewhere in the system where it's needing a circuit completed?
I confess, this has us stumped here and could use some input on a proper troubleshooting method. Sorry if there is some obvious electrickery here that I'm completely overlooking.
And I'll confess that I hate trying to diagnose electrical problems remotely, because too often relevant information isn't provided because the poster thinks it's irrelevant. But, I glanced through your post and, if you look at the 2nd wiring diagram at the following
it appears that the issue could be due to the coil (or connection to the coil) at the right in the bottom row of the hand-drawn stator diagram. Four sets of resistance measurements are typed below the wiring diagram -- what are the values for your stator?
You are the man, Mr MagnetoMan. This is exactly what I needed. I do have that wiring diagram (or very similar), but it was the ohm values that helped me track the problem - a cracked red wire in the sheathing.
As an aside, my values are as follows:
Brown - Black/White = 4.4 (your value 4.3) Brown - Red = ~.5 (your value 0.42). But this was no value until I traced the crack (thank you!) Brown - Brown/Blue = 1.1 (your value 0.81) Black/White - Brown/Blue = 5.0
I connected it all up as it should and the bike started, ran beautifully, and all systems work perfectly.