When i bought my Newport 6 years ago one of the first things was to have the heat exchanger cleaned and repaired. material from the pencil Zink can build up inside and residue that gets past the strainer will stick in there. I also replaced the thermostat. A mechanic that used to work for Capital Yachts in Harbor City indicated that the cooling system is critical on the engine.
One of the prior owners thought they new more than Gary Mull, boat designer, and put a different pitch of prop from the factory unit. The engine would not warm up, and left black soot all over the transom. Correct prop and bingo, problem gone. I now get 135 degrees upon leaving the harbor, 150 to 160 degrees at 2300 RPM.
Starry Night -Oxnard Calif