take lid off cistern , adjust ,the little plastic nut at the top of the ballcock arm , you will see it up near were the water comes in , flush the cistren first. let the arm drop, turn the little nut clockwise, let the cistern start to fill , start adjusting the nut, till it turns the water off just before it reach, the overflow. ps, there might be a little lock nut on the adjuster screw, that you will need to open first before it will turn , lock it back up after you have adjusted the screw, its there to stop the screw from going in or out, hence" your problem...
mick
April 2008
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