The poppet valve should be checked and cleaned too. I don`t understand your ? Motor should idle in gear in the lake at 6-700rpm`s. On a cuff it will idle 900-1000rpm. These Mercury`s are a low pressure cooling system.
“The world will not be destroyed by those who do evil, but by those who watch them without doing anything” Remember what He has done and remember that He is not done yet! GOD IS GOOD