I've never had any trouble running an outboard on the muffs. I'd try
adjusting the pressure, if I knew that the water temperature was
getting warm enough to open the thermostat.

I have solved the problem by running the motor in a small water tank.
With this setup, I can see the telltate water stream coming out that I
could not see when I used the flushing earmuffs. And the motor
temperature didn't go over 130 degree when I ran the motor for 10
minutes.

I still cannot figure out why the flushing earmuffs cannot give me the
desired result. May be I should have increased the water pressure to
almost full pressure. But I don't want to risk damaging the impeller
in the water pump. Moreover the small water tank is _much_ cheaper
than the flushing earmuffs ($5 vs $30). Seem like I am better off
using the small water tank instead of worrying about how to get the
flushing earmuffs working.

Jay Chan