Join Date: Nov 2009
Location: Lander, Wyoming
Once the fuel tank is removed, the source may be quickly spotted. I'll guess the RH forward valve cover mounting bolt & its rubber gasket.
Even if the source is readily found, plug the fuel hose & fuel tap vacuum hose and using a degreasing product (like Gunk Engine Brite) and water to rinse (Hot water if you can), proceed to thoroughly degrease & de-mud the entire bike. Especially the top of the engine & around the spark plug. There is spark plug well drain hole below the thermostat housing, use a wire or even a drill bit to clean it out.
With a thoroughly clean & DRY engine, if necessary one can spray a white or gray drying Cheap Foot Powder around the suspect area. It will stain very quickly upon start up to show leakage area.
Modify at "YOUR OWN RISK"!
Still riding my 1987 KL650-A1. 85,000+ miles & counting