I have an 87' yamaha 30 HP and it keeps over heating if i let it idle or run it slow. there is a little red warning light that turns on and then if I try to get to speed it wont let me, it will keep kicking and bouncing when i get to half throttle. It has never had a very strong pee stream since I got it. I have replaced the impeller with no effect. any suggestions? the weird thing is I can drive full throttle all day with no problems. I assume this Is because the impeller is moving faster and pushing a little bit more water there for cooling the engine enough.

Sometimes just the impeller won't do. Replace the whole assembly and seals.