1983 XL80S

[67Camaro]

Working on a friends XL80 as it wasn't running quite right. Low speed around the house is ok but at higher speeds it cuts out. Pulled the carb and cleaned it thoroughly, sonic bath overnight followed with carb cleaner. Voltage is an issue as the new headlight worked well until I ran up the rpms. Discovered the voltage is spiking. Cannot find a voltage regulator. It is called out in the wiring diagram but nowhere to be found on the bike. My question is, would the lack of voltage regulation result in the poor high speed performance of the motor? Granted this is a 37 year old bike. Thanks for any help!