Thanks for all the info, indeed I have a OM616 with 67HP. Indeed some marine engines do excist counterrotating, but mainly engines where the timing gear allows this, like a standard good old American V8, and some V6's . In this mercedes engine reverse turning would destroy timing components, so both engines do turn the same direction. Sometimes The gearbox allows for a revese turning prop by changing forward and reverse controls, but anyhow this is outside the scope of this forum. The fuel issue I did adress by using the fuel source from the good running engine , there was no difference.
I do have to say however that the fuel that the bad running engine uses was very very red, compared to the other engine that was more normal colored.
Now in the US we add a red die to fuel used for off road application, but this boat was converted to diesel and only went to the diesel pump once. The fuel that went into the both tanks was the same. So I am curious about the very red appearance of the fuel for the bad running engine, it had a varnishy smell too, which makes me believe that maybe it is trying to dissolving old gas that might be left in the tanks. Anyhow I did change fuel source, and there was no difference.
However maybe the mix of gas and diesel migh create havoc and dissolve seals in the pump?
Anyhow, I will send the pump out for inspection.

Henri.