This discussion reminds me of a Uniflite that I once sold. One engine ran
pretty badly, and one ran well.

We discovered that one of the fuel tanks was quite a ways aft of the engine, so
far in fact that the normal fuel pump was overtaxed drawing fuel from that tank
to the engines. A previous owner had installed two additional, elelctric fuel
pumps between the tank and the engines, and it turned out that the auxiliary
pump associated with the engine that was acting up had failed. The engine got
enough fuel to idle, but would *never* achieve any kind of speed without
starving out, hot or cold.
Put in a new "booster" pump, and the boat ran sweet as kittens thereafter.

Be sure you let us know what it turns out to be, once you have tracked it down.
:-)