I have a comment on your problem. The top of the water pump housing needs to have water in it to work properly, I have had a couple of these motors that the water outlet was modified and all or most of the warm water was exiting the blow hole. My 18 would send steam out when I was moving slowly then when I would speed up it was OK. I pulled the power head and a PO had hogged out the water shoot with a rasp or something. I found a washer just the right size and tapped in the port so some water would be directed into the exhaust. I know clear as mud! You do need some water splashing around in that housing. That might keep it cooler.
dale
