I have a 3 hp mercury. It is a 2003 i just changed the impellar and it pumps water great until the motor starts to warm up. than it is steam out of the pea hole. I am running j4c's.
My 5hp Yamaha had the same symptoms. Impeller was changed so I pulled the exaust cover. It was full of corrosion and 2 water ports were clogged. I poked through with a coat hanger and now runs great. This is just one possibility. You should check the compression first to make sure nothings going on there.
