It seems that everyone has a problem getting the spark plugs out of these bikes. Usually, the 18mm socket is too thick walled. Well, I found a socket that works excellently.
I went to Sears, and found an 18mm Spark Plug Socket. Hard to find, but it works wonderful. Easily takes out all 4 spark plugs, no clearance issues, with the valve cover, cylinder head, or frame.
The only bad part is that it costs like $4. Expensive for one socket. It even has the slotted (?) sides so you can get a crescent wrench around the top if there's no room for a socket.
Doug