If you have a bike that is pre 1976 (77 in Canada) it probably did not come with a regulator. The system of the bike uses a charged, good condition battery to manage variable output from the stator. If your battery is bad, or sometimes even just have a burned out taillight, you are in for an expensive headlight replacement. The solution is a regulator. Yamaha recognized this and published their Service Bulletin #M5-084 re Voltage Regulators Which explains both the problem as well as the solution. The new problem is that an NOS Yamaha regulator is EXPENSIVE.
The best solution is probably an Ebay replacement off of a later bike. It needs to be 6 volt. The Service Bulliten explains the installation process. At $30+ for headlights, it will pay for itself quickly.
We are also in the process of testing a new regulator which may solve the problem and will update this FAQ as more information becomes available.
