7.4 merc running warm
 

I am working on a 1990 Sea Ray with a single fresh water cooled 7.4 merc and bravo drive. I am having an issue with it running warm under a load. The motor will get to about 190*. However, the sea water exiting the heat exchanger is only 105* and the risers are at 115*. I have replaced all bellows and hoses on the outside of the boat, water line from gamble to sea water pump, sea water pump impeller, exhaust manifolds, risers, and thermostat 143*. Flushed and cleaned the heat exchanger. I am clueless as to what could be causing it to run warm. Any advice would be greatly appreciated!
Thanks
Paul