That seperate lighting coil in the 87's bypasses the battery and runs the light off a/c current. The faster the engine goes the more power is sent to the light. There is a 12 page thread on the topic in here. Yamaha realized this system sucked and changed it the next model year.
You should be seeing 5vac at idle and 13vac at cruising speed
User mcc1776 if I remember right, put in a rectifier and a 20w dc led headlight and was happy. There are some led lights that are compatible with a/c current.