'59 R50 loses electric at temp

[remmettlee]

Hey all, looking for some insight to a problem I am having on my 1959 R50. I have only had the bike for a month, and have been resolving some issues, e.g. timing and points, but took it on a long ride over the weekend, during witch everything was going fine for the first 30 min, then the electric failed...no lights or horn, no indicator lights, nothing. After letting it sit for 15 min, went back, put in the key, and violla! indicator lights working! Started it up, all good. Then electric failed again. Now I have electric when cold, but as soon as the bike warms up, electric is gone. Any similar experiences or best ideas on how to isolate? I have all the shop/repair/restore books, but can't find anything similar in troubleshooting so not sure where to start. Appreciate any help that is here!!

[Pauls1150]

schematics at https://bmwmotorcycletech.info/Slash5Schematic.jpg

I'm not familiar with the older bikes, but when "everything" dies, that points to a "primary-side" failure: battery, alternator, rectifier, regulator...

[20774]

At highway speeds, power comes from the generator. At idle, when generator output is minimal, power to the lights and horn comes from the battery. So when you get the power failure happens, does the ignition cut out? Do you have to pull to the side of the road?

My first thought is that the original magneto coil is failing. A classic sympton of that is when you go to a gas stop, the bike is hard to start after filling up...hot engine. But after waiting a number of minutes it starts fine. The coil is just one long wire. Problem with the old coils is that breaks in the wire happen, resulting in a lower quality signal.

[69zeff65]

Take it out for a ride and when you lose electric pull over and stop, leave the motor running and try pressing down gently on your ignition key (the black plastic one in the head light switch) if your lights and power come back on you have narrowed it down to either a worn out switch or the switch plate inside the bucket has loosened up.