From the FAQ's.
Dan


Adjusting the Tensioner After Installing a New Belt. [Stoney] I just got email from 2 dealer techs I know, with 18 and 20 years experience between them both are top VISTA techs and they said that the belt should be adjusted after 5-600 miles due to the fact that a new belt will stretch some in the first 1000 miles and they have seen a few jump the cam gear..
[Response 2: Don Foster] The tensioner has a spring in it which forces the idler against the belt to a predefined tension. But then, when you tighten the nut on the tensioner, you lock its position. The instant the belt stretches one hair, the tension is relaxed (because the tensioner's position is fixed, it can't move inward to maintain the force). The tensioner, in this configuration, compensates for (adjusts for) the increased length from stretching -- to prevent it from jumping a tooth. It DOES NOT maintain a constant force on the belt. It's assumed a new belt will relax a bit during when first used, so you must loosen the nut, allowing the spring to again reposition the idler. This consumes the extra length from stretching -- but then you tighten the nut again, locking the idler's position. Ideally, there will be virtually NO tension on the belt, but also virtually NO slop in the belt. Hence, after about 500 miles, more or less, remove the rubber plug from the timing belt cover, loosen the tensioner locknut, rock the crank slightly clockwise about 1/8 turn (this makes sure that belt has tension on the drive side between cam and crank gears), and tighten and retorque the locknut. [Editor] Make sure you lock that tensioner nut to prevent the tensioner from loosening and destroying the new belt. Torque to 50 Nm (37 ft-lb).