The regulator is pretty straightforward, before you begin take note of where the regulator is set. Pull the housing off the engine, back the regulator screw the whole way out and remove it and the spring, noting which way the conical spring points, and flush it out with raw fuel while exercising the plunger. Make sure you do all this in a clean environment as any dirt in the area will find its way into your regulator and cause you grief.

If you are not comfortable doing this you can box it up and send it in to YS for repair. Given the age of the 91 it might prove prudent to have it gone through.