Volvo Penta D1-30 smelling like burned plastic
I just noticed that the Penta D1-30 I've got in my Bavaria 37 smells like burned plastic at around 1500 rpm or more. My normal cruise rpm is just under 2000 and when I'm in a hurry I can run it for short distances up to 2300 rpm. Now it refuses to do more than 2000 rpm and the exhaust seems blacker than normal.
It starts without issues, everything looks like it is tight and clean. Sea water filter is clean. Fuel filter tight and recently changed with the oil as part of the annual service.
What could be wrong and what should I check?