I have a copy of the Supplemental Manual, but I don't have the 640 "base" manual. The supplemental manual is very vague on how to check the valves, so I started Googling and found this site. In particular, Hurricane posted... ".Remove the seat,tank,upper engine mount ,cooling fan motor,spark plug wire and temp sensor wire,then valve cover.Remove the two plugs on the left side of the crankcase and turn the engine with a socket over counter clock wise thru the bottom hole on the crank till the intake valves open and close then go past that approx 1/3 turn and line up the letter " T ".It lines up in the upper inspection hole.Adjust all valves at 0.23mm and button it back up." in this thread from '10 http://klrworld.com/forums/index.php?PHPSESSID=097ac6ccbfbbccab950533669e3a6a19&/topic,16326.0.html
Is it REALLY that simple? That doesn't sound too bad. And how about "buttoning it back up?" Any special procedures there? Do you have to put Form-A-Gasket where the head and valve cover mate to make a new seal?