I have a Red Lion convertible shallow well jet pump. It is set up to pull water from a lake, about three feet below the level of the pump, through a 1-1/4 polyethylene line. Other than a vacuum leak at the inlet barbs that I am working on, I think everything is working okay.

The discharge goes uphill against a 20 psi head to the main building where we want about 40 psi. From there another 15 psi uphill is a holding tank that supplies water when the generator that powers the pump is shut down.

This pump (and probably many like it) has as valve or adjuster next to the discharge, opposite the priming hole. The manual describes this as a "control valve", but says little more about what it controls, how to adjust it, or why.

Why would I want to mess with this, and what would I do to adjust it optimally? My guess is that this allows me to make a volume/pressure trade off somehow. Where can I look to understand the ins and outs of messing with the control valve?