Firstly remove the top casing and then you'll see the retraction system. It is similar in design to a lawn-mover pull start!! There is a wound, flat spring on one side which provides the tension. Tension needs to be re-applied to this spring to make the cord retract and to do this you'll need a little socket set or spanner. Wind the cord fully onto the spool, loosen the nut holding the spring in place and turn the spring housing until you feel the tension. Tighten the nut and test cord retraction. You may have to do this a couple of times, mainly to ensure that the cord can be pulled all the way out, and that the tension isn't too great otherwise you'll end up with other problems.
If you can, replace the upper plastic coating on the roller with a piece of quality nylon tube because the factory supplied plastic wears very poorly.
Piers40
October 2009