Adrian.pull the motor back as far as you can so there is loads of tension on the spring.it will be hard to hold the motor back but use something to hold it back.now fit the belt over the motor shaft.belt should now be around the drum and around the motor shaft but loose.now let the tension on the motor go and the black idler /tensioner wheel will put pressure on the belt and tension it up.job done.
ck
March 2009