I recently had a new problem crop up and have been monitoring to see if I can be sure how it happens on my 1978 R100/7.
Here are the occurrences:
About 1 month ago:
I started out riding and was about 2 miles away from home heading away and when I turned on my right turn signal at an interstate on ramp, my bike for a split second just died - I quickly noticed that all the dash lights came on (like when you turn on the key before starting) and the engine quit firing, but in the split second, my natural reaction was to turn off the turn signal and everything turned back to normal. I purposely tried the signals, brakes, and other electrical items from then on, everything worked fine for the rest of the trip. I feared I might get stranded somewhere further away, so I cut short my ride to about 10 or so miles. No more issues after that!
About 2 weeks ago - second instance.
Knowing the above had happened, I now, each time I ride, check the operation of various electrical items (horn, brake light, signals, high-beam flasher, etc.) early on so if there is a problem I can catch it close to home. This time, when I hit the brake out by the stop sign just a half a block from home, same thing as above happened - i.e. dash lights came on, and the bike immediately quit running, but I was going so slow, it didn't re-start when I got off the brake. I merely turned off the ignition switch, turned it back on and bike started normally. The situation never happened again for the rest of the trip - dozens of miles with lots of electrical item usage - brakes, turn signals etc.
Before this first instance happened as described above, I had ridden this bike around 4,000 miles with no such problems. Between the above rides, I had ridden many times but no problems.
After the second instance above, I had ridden many times - no issues until today.
Today - third instance. I rode the bike, using all the electrical items, for about an hour stopping for breakfast, going to the hardware store, and eventually up to a car dealer to purchase trans fluid for my car - about 20 miles away. Then just about 3/4 miles from home, I was getting off the highway on a small off ramp, I turned on the signal and the situation happened again, but probably because I was also using the brake at the same time, I didn't get things "turned off" for abut 50 feet or so, the dash lights came on and the engine quit, but before stopping completely, everything returned to normal. I then "experimented" by trying different items and unlike the first two situations, this time the bike quit and dash lights came on each and every time I tried anything electrical. This time I came to complete halt as I wanted to be sure to get out of the way of any traffic. I turned off the ignition switch, turned back on, bike started normally and everything worked fine after that - even when I tried "experimenting" with each of the electrical items, brakes, turn signals, headlight on, headlight off, horn, high-beam flasher, and NOTHING would duplicate the failure - bike kept running fine!
I am hoping that there is someone out there that knows of a simple issue that could cause my problem (like alternator brushes need cleaning or replacing or something else) and not make me have to go through all sorts of guessing.
A couple of weeks ago, my son's K-bike starting having electrical lights flickering and the simple fix was common to the bike. The users knew exactly that his problem was - it was his starter brushes needed cleaning! He did it, and it was the proper diagnosis/fix.
I am hoping there is such a situation like his with my bike. Just some further info.: The only things changed on my electrical system that I know of are these: previous owner had soldered in newer-style in-line fuses to replace the original fuses, and 2) I wired in a simple rocker switch to turn off the headlight (as older bikes had) so that I could turn on/off the head light at my desire. I have ridden literally 4,000 or more miles since I did this with zero problems.