Thanks for the reply. I'm not sure cleaning the bulb contacts will make a difference since the signals work as they should when the 4-way flashers are engaged but I'll give it a shot.

The problem is most likely caused by corrosion on the electrical connectors, most likely on the contact board inside the headlamp. Here is a portion of the response I gave to another electrical question....

My 1979 R100 had numerous connectors with corrosion on them. In fact the horns didn't even work. It's simply a result of 35+ years of water and humidity doing its work on brass connectors inside the HL shell and under the fuel tank. My corrective action, which cured all my electrical problems, consisted of individually unplugging every connector and relay and adding "electrical grease". The compound used is called "No-Ox" and is available on Ebay and Amazon. No-Ox is no ordinary grease; it is not a lubricant. It is a specially formulated paste to remove corrosion on electrical connections and prevent the oxidation caused by water. This stuff really works and it takes only the smallest amount, about the same volume as a match head, per contact. So the smallest $2 tube is enough for 20 motorcycles.

When this work was completed my headlamp was brighter, my horns started working, and my battery voltage stabilized. So I highly recommend this action as your first step.

Hope this helps.

Thanks all for suggestions.
The problem's been solved; it was the flasher relay. It was difficult for me to find the problem because various wiring diagrams showed different arrangements, different color codes and weren't all inclusive. The introduction of the hazard flasher in 1979 added a second relay. According to Snowbum this additional relay "is the only one besides the flasher relay that interrupts the power to the flasher if faulty".
Snowbum provided additional wiring diagram for the hazard flasher relay which is at http://bmwmotorcycletech.info/BMWHazardCircuit.pdf

Rick