Re: water pump test????
I think that one has 4 or 6 small holes in back where cooling water is pushed out by exhaust gasses. There may not be much mistin out, and none coming out at slow idle and your motor could still be cooling correctly. Test in a barrel, with the water level OVER the water pump (not self priming) and run it. Idle slow and touch the base of the spark plug or next to it on the cylinder head. You should be able to keep your finger there for a couple seconds. If it gets hotter, and you splash some water on the head and it boils, you are overheating. If you can idle slow for 10 minutes and it doesn't overheat, it's functioning. Even w/new impeller, you may only get a spritz out of those holes at low idle. Just make sure it's cooling and go fish!
Good luck.