I suggested before it was probably vapor lock. Seeing gas in the
filter does not matter, the vapor lock could be in the carb or in the
line elsewhere. Take a pure co2 fire extinguisher along next time and
give the carb and fuel line a squirt to cool them down and see if it
runs normally again.

JV wrote:
My '85 4.3L Mercruiser starts losing power at plane and starts surging
heavily at idle after about 30 minutes of running. The motor is not
overheating and the clear fuel filter close to the carb is still full of
gas. Any suggestions on where to start?