If you are at all handy,it's dead simple to check and or change the belts.The valves are just as easy to check,but a bit more complicated to swap shims.Chris Kelly is worth listening to in these vids.
[video=youtube;oXUgtytXI9o]http://www.youtube.com/watch?v=oXUgtytXI9o&feature=player_embedded[/video]