James Gemmill wrote:

Elevation of test, about 4000 feet above sea level ...

At that altitude you are producing considerably less than the rated sea
level power output to begin with. If the temperature of the intake air
is much above 45 degrees F you will lose even more.

The chances of burning exhaust valves at reduced power are pretty slim,
especially if the engine was not adjusted for altitude and is running rich.

Does the boat live at this altitude or did you just take it up to a
mountain lake for the weekend and found the performance less than you
expected? Is your mechanic based at sea level or a local guy on the lake
who is familiar with local conditions and boats and knows your setup?

Rick