With a positive displacement pump, the ball valve won’t reduce the flow unless you reach the psi your unloader is set to, it will just increase the pressure, and it won’t draw chemical. The pump produces XX gpm @ XXXX RPM, no matter what. unless you exceeding the set psi, then it bypasses. Think about it, it pumps the same amount of water through that tiny hole in your tip and it does through an open hose.
Also, reducing the RPM is no problem at all at low pressures. But not good at all at high pressure. A gx390 produces 13hp at around 3200 rpm, at 1500 rpm is only produces about 7hp. The problem comes when you let off the trigger or close your valve at low rpm, the pressure tries to rise to the set psi, so it can bypass, the engine doesn’t have enough HP to make that much psi, and it stalls instantly. This is VERY BAD on your lil Honda!!!