I have this same Mercury I have been working on for a while now. It was a complete rebuild. Bored 15 over new pistons and rings and bearings. I have also rebuilt the poppet. It is running good but it is running about 20 degrees hotter on starboard head. This is while on the hose. The port head is about 143- 145 but starboard is close to 160. This is being measured with infrared thermometer. I am going to pull lower off and check impeller. What else should I look at?