Hola.

Try this: just with the weight of the motorcycle, and in neutral, move the bike foward and test carefully with your finger to find the point where the chain is most thigh. Adjust the chain tension in that point (5-6 cm slack on the XT660Z) and then see how tension works. Be sure to have someone helping you to hold the bike while you move it around so you do not drop it.

Anyway, I have found that trying to find the right tension with the rear tire off the ground is not very accurate; what is more, XT660Z user�s manual illustrates the procedure as I said above.

I hope this helps