Pull your thermostate. It is located just above the top spark plug. Be careful when taking the cover plate off. There is a spring behind the plate, so back all 4 bolts evenly until the cover comes off.
Here are a few picture.....I have a 40 hp Force engine.
If your thermostat is dirty, after you wash it off, put it in a pot of water on the stove and bring it up to a boil. When you start to see small bubbles on the bottom of the pot [ie. ~190 F], the thermostate should start to open. When the thermostate opens, it will free up more "trash" and clean it much better.
If it does not open.....buy a new one, for ~ $ 25. I have used the same thermostate for 12+ years and have pulled it many times to clean it and put it back in.