I have a Kohler Rialto one piece toilet that occassionaly does not have
a good flush. I believe it is because the bowl does not fill with water
completely after a flush. I have adjusted the float, and the flapper is
working fine. There is a diverter valve in the tank with a float, but I
do not believe it is adjustable. Then I noticed on my ballcock valve
there is a plastic adjusting screw on the side. Look at this figure.
The screw is on the right side:

http://www.plumbingsupply.com/images/fillvalve-1b1x.jpg

What is this screw for?