Hi Bob, I was having issues all last summer with my mariner 4 not producing the typical amount of cooling water from the output port, and fyi, it should be a small steady stream or slightly intermittent in phase with the cadence of the motor with none exiting the exhaust. Pretty sure the small droplets of water we see from the exhaust is just condensation, anything more or excessive and I would be tracking the source.
I use my 4 HP as a kicker on my sailboat in fresh water lakes. Like you I pulled my lower unit a few weeks back, and I'm no expert, never even seen the inner workings below the pivot and clamps...but everything looked fine, so I cleaned all the components and reassembled. Next I tried flushing the cooling system, which can be a less than convincing task without an adapter to connect a garden hose...so I decided to pump vinegar through the cooling system using my water flosser, normally for my teeth, but it has a decently pressurized output stream which actually works quite well! I inserted the flosser's attachment into the water uptake tube above the impeller housing, as well as the tell tail port under the engine..nothing changed. As a professional scientist fairly adept at chemistry I was thinking about diluting my lab HCl, but couldn't bring myself to subject her to that quite yet...Finally after talking to my stepdad I borrowed his large air compressor with this custom long narrow brass tube attachment he uses to remove gooseshit and debris from his boots and immediately I heard something dislodge internally. When I started it back up in the trash container with water it quickly cleared the system of the impediments and produced a nice steady stream. I assume it was insect larvae, probably pupa casings. So from now on it's winter inside or if I have to keep it outside there will be a tarp wrapped tightly around her.
You mentioned running your 50 hp onto a mudflat and I immediately thought of my insect larvae theory, but in your case benthic. All sorts of creatures reside in the upper benthic zones of lake/river bottoms and most definitely in our oceans/estuaries..just something to think about.
Mike
