Whoever made your raw pump should have rpm/volume/pressure curves.
Some engines have a valve to bypass some of the water directly overboard
. You only need enough water to help muffle and cool the exhaust. My mains are setup this way. Too much water in the exhaust will create excessive back pressure and hurt economy.
A pump with a worn impeller can't push as well against increasing back pressure.
My 4108 is a generator
engine and above the waterline. At idle, the water is more noticeable than @1800 rpm when it's more spray. You could have a marine
buildup somewhere that still allows enough water thru at low rpms.