I use both the Maruyama and Solo engine drive backpacks for pressure regulated spray applications. The Echo has a fixed orifice on the line back to the tank. I would not touch that. What I would do is put standard 1/4" stainless steel flow through quick couplers as is used on pressure washers. Then I fit a Watts 263 adjustable regulator between the hose and the spray wand. You can use any of the Teejet hand pieces once the hose is converted to 1/4" quick couplers. Set up this way, I am able to make precise and controlled applications to ornamentals or lawns.
They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety.
Benjamin Franklin 1775
Sell not virtue to purchase wealth, nor Liberty to purchase power.
Benjamin Franklin Poor Richard's Almanac1738