With my 1993 Merc 150, water doesn't come out of the pee hole until
2100-2200 rpm, using the ear muffs, even with a brand new impeller.

Sound like this is a common problem with flushing earmuffs adapter, and
I will only run the motor at idle speed (around 1000 rpm) when I am
flushing the motor or running the motor to winterize the fuel system
anyway. Therefore, seem like this flushing earmuffs adapter is not
suitable for my motor, and I should be using a small water tank. Oh
well...

Jay Chan