Hello Lilly,
You are probably talking about the stop motion wheel that is in the balance wheel, if you have tightened it fully and it is not gripping then you will need to undo the small screw on the stop motion wheel, undo the stop motion wheel all the way off, turn the three pronged washer round into a different position and then put the stop motion wheel back on, tighten it up and put the small screw back it, try it and see, and if it does not work put the three pronged washer in a different position, fairly simple.
any problems contact me drivewithken@blueyonder.co.uk
Ken
drivewithken@blueyonder.co.uk, October 2007