You are unlikely to do any damage, tight valves are much more of a concern than loose ones.

The job is a fairly easy one, as the valves are screw and locknut type adjusters. The radiator can make things awkward on the exhaust side, if you remove it's mounting bolts and let it fall away a little, you can buy yourself a bit more room without having to drain and remove it. There's a bit of a knack to removing the tank; watch the fuel pump/sender assembly its fragile and easily bust (chock it up on a block of wood at the back to create room to remove connectors) and watch the tank doesn't get caught on the radiator cap.

Get yourself a copy of the Haynes Manual - it'll be worth the investment. Or become a XT660.com supporter and download the Yamaha workshop manual. (See Kev's post above for link)

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