Hi Bisquit,
Thanks for the response - I know it's a common place for the tank and would probably be fine but it just gives me the hebejebes! What has your experience been with with fuel consumption? Ever have issues with algae in the tank?
Pulled the head off late last night and everything looks good. Cylinder liners are in great shape with cross hatch still visible and no scoring or uneven wear apparent. Valves looked good too. A little carbon build up on everything but nothing that would cause the engine to not run or even run rough. I lapped the valves since I had it apart anyway and cleaned all the carbon that was accessible. Going to order the gaskets and get it back together soon.
The thing I was not expecting was the water injection elbow (it's the u-shaped one in this case). On the outlet side it was very corroded but had a fair amount of area for gasses and water to escape as you can see in the picture. After much heating with the oxy-propane rosebud the inlet side came apart (meaning separated from the exhaust piping that is between the water injection elbow and the cylinder head, what would be called the manifold if it had multiple ports) the interior had closed down to less than a pencil's width for the exhaust to escape! It was impossible to see this until the two had been separated. In the photo you can see a greenish area that is the passage.
So my thoughts now, after seeing the inside of the engine looking quite normal, is that the exhaust had closed down to the point where back pressure was extremely high and thus the engine could not pull in the fresh air charge. This made the fuel to air ratio very rich for any given engine load and caused incomplete combustion, giving the black smoke that Mike experienced back when she conked out. The only thing this doesn't explain is why the mechanic reported low compression on inspection in the boat. I should have tested it before tearing the engine down but I was just so convinced I'd find a smoking gun in the valve train or scored up cylinder liners I just went for it - lesson learned.
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