The same query came up a few days ago.
        
 
         Have a read of this post:
         
         
        
http://www.xt660.com/showpost.php?p=204583&postcount=32
         
         It'll save me explaining it all again! 
        
         
         (Just ignore the bit about the O2 Controller if it's a pre-2007 bike - the rest still applies.)