Prizm at full throttle.

[beaker020]

all you've done by increasing the head (actually; lift) is reduce the flow just the same as if you were closing the valve. The skimmer doesnt run any better, it just makes you feel better because the valve is open like it 'should' be. A better approach is to make a short riser for the collection cup out of PVC or ABS. This will allow you to run the valve full open AND get full flow at the same time.