I have twin 306 HP TAMD61s on a 32 sportfish with <1,700 Hrs. My port engine only makes power up to 1,500 RPM at 40% throtle. Anything beyond that the RPMs stay the same and it produces increasing thick black exhaust. Out of gear it revs fine with little smoke. I've replaced the fuel filters and am guessing that air management is the next place to look? Either air filter and/or turboperformance need to be checked. Any ideas on what else to checkor procedures I should follow? Need to get fishing!

clean the air filter asap