Sounds like you can use a shop book. It's a good investment. Being a rider makes a lot of people into mechanics, specially when owning British or American bikes. Yamahas are pretty dependable and tough.
Do not tinker with the engine without clear and complete information, or take it to a shop for a full tune-up. Fuel quality might have changed the gas mileage, but you can expect things to need replacing or adjusting after 18000 miles. Clean the air filters and get new spark plugs and see how much a difference that makes. That can't get you in too much trouble. Gas mileage can be adversely afected by worn out tires.
I don't know if the valves are adjustable, but a good shop book will show.