Hello again folks!
Well, this is it! I am the owner of a 2006 XT660R. The bike is in good condition, except for the tires (Metzeller Tourance do not suppose to be square slicks, right?) and the spark plug, which has black electrodes. We will see if this is because of a too rich air-fuel mixture (maybe following-up the modification of CO value in order to reduce the surging effect) or because of the cheap spark plug itself.
The bad news is I am not able to play with it in the following period of time, first because I don't have the registration plate yet, and second... the winter finally came in my country also.