JV wote:
After about 30 to 40 minutes of running at 3000RPM, my 86 4.3L 205 HP
Mercruiser starts losing power and when you slow down, it surges heavily and
stalls often. I have a clear gas filter in the line next to the carb and it
is still full of gas. The engine is not overheated.

Any ideas?

I have a 5.7 mercruiser that may have had a similar problem. After
much frustration and wasted effort (changing the water separator
repeatedly, draining the fuel, flushing the system, blah blah) I met a
guy that had the solution. He told me the fuel line insulation had
been removed and it was vapor lock. I thought he was crazy but just
placing some styrafoam type insulation like you see on water pipes has
solved my problem for over a year now. I don't know if it applies but I
hope it may help.

Jerriko