The exhaust valve on my Saito 72 is sticking. It's intermittent and always frees-up after starting. The engine has 4 gallons of CoolPower 15% through it. I never run it too lean. The engine is mounted at an angle such that the exhaust valve cover is a low point. The valve stem has considerable black build-up as does the muffler and header pipe. The intake valve looks new.

How do I fix this and how do I prevent it?