Symptoms don't necessarily sound like corrosion issue---could be fuel pump issue / VST issue. VST can be drained and cleaned. Here's a post I found on another site: Might give you some ideas on where to start looking----
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last October my 2004 F-225 lost RMP while leaving the dock. When I advanced power lever to 2100 RPM it would just bog down. Called a mechanic and ran the boat at the dock. Put it under load and it worked fine. Left the dock and got 6000 RPM for almost an hour. Then RPM dropped off and and boat would barely run above idle. 1/2 hour later boat ran fine. When we put it on the trailer it would not get above 1500 RPM.
Mechanic said VST screen was probably clogged so we removed it. it was full of white chalky stuff. Cleaned the VST screen and boat ran great for 14 hours. Same problem returned but have more depth now. When accelerating, sometimes at a RPM range near 2000, or 3000 or 5000 the boat suddenly bogs down. If you continue to push up the throttle RPM decreases but when you pull the throttle back, at some point RPM increases again. When it happens engine does run rougher than normal but one minute later smooth as can be at 6000 rpm. Put a fuel gauge on the rail pressure at 42 lbs then suddenly drops to 20 then boat runs real rough at 10 shuts down. Ran on an external tank same problem. Have done diagnostic several times no problems show up.
Took the VST screen off again and found it full of white powder. Found the bottom of the VST tank corroded. replaced the VST tank. Same problem. Took all fuel (clean) out of the tank. Replaced pick up and fuel tank indicator. Replaced all filters on the engine several times, replaced racor several times. Changed the ball, All has had no effect on the problem. last week decided to change the HP fuel pump. Pump looked good, clean could not get any deposits out of it. Finally ran the boat today and the problem is the same. Seems to be totally starving for fuel (sometimes). Initially would not get above 1500 rpm then 2200, then 3100 then suddenly it accelerated normally and got 5700 RPM and ran smooth. As soon as I pulled it back it would not get above 2000 again for the rest of the trial.
The real symptoms, run the motor in a bag. looks and runs smooth. Put it under load and it seems at some point (low RPM) to start running a little rough. Advance the throttle and everything feels normal until a certain RPM and then RPM decreases rapidly and engine starves for fuel. Bogs sometimes backfires or just sputters. Pull throttle back and at some point RPM increases to a point.
I guess in re-thinking the problem, most probably the severely clogged VST screen was probably never the problem or it was just part of a problem. The only thing left is the fuel regulator and I'm not sure if this would have these symptoms or not. I think having the pressure drop on the rail eliminates injectors, injector pumps and the like. I think. Of course today it ran at almost full power without skipping a beat for several minutes.
I now have been given the idea to pinch the return line when it happens to see if the regulator is porting too much fuel. I'll try that next time I can out the boat in the water.
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One of the respondents to this suggested also looking at the VST float valve. Have a look at boats.net ---you can find the entire micro-fiche of the VST of your outboard, plus all the part numbers
good luck