My sprinkler heads only spray about half the required distance, too
little water pressure for head-to-head coverage, despite careful
caluclations during the design phase. However, when I loosen the
solenoid on these old valves, voila! Works just as it should! Is this
an indication of faulty valves? Would replacing the entire valve
likely solve the problem? Perhaps just the solenoid?

BTW, if I leave the solenoid loose, the valve doesn't shut off, so
that's not an option. :/

-Fleemo