No t stat on that. Try sealing the impeller plate down to the casting with some 3M 847. Check the grommets, both upper and lower. IF swollen, they will prevent flow. Also, ensure you grease or oil them before installing the gearcase to ensure she slides up smoothly. Is the vent hole in the impeller housing clear and not blocked? Housing badly worn? Intake area free of debris?
Was he running it in a barrel, and if so, was it deep enough?