Re: 1976 evinrude 15 hp
If you have not changed the impeller within the last 5 years, or you don't know when it was last changed, you need to do that for sure anyways.
That being said, you might try reaching around that motor towards the exhaust relief hole, when it stops spitting water, and see if you can feel the water, it is probably there. That motor does not have a tell tale, but has an exhaust relief hole where the cooling water is supposed to come out. It is designed so that the water comes into the exhaust cavity and flows down the exhaust chamber and out the exit holes, lower on the skeg. In order to observe that the pump is working it's design made some of the water deflect out the upper exhaust relief hole.
So when your pump is working perfectly some of the exiting water is deflected out that upper hole. When your pump starts to weaken, that water angle gets adjusted and because this deflection is now hitting the edge of the hole, instead of the hole itself, it just dribbles down the back of the motor at high speeds and is very difficult to observe, when just looking back at it. You should be able to observe this by looking back at the spray when at medium throttle and as you increase the throttle, you will see that water angle slowly move towards your motor. As you decrease throttle that water angle will move out away from the motor.
To exasperate this condition, that motor also only shows a good spray when the T-stat actually opens (can usually only feel a mist before that). Since I have to assume your water is a little colder these days, as is my water, perhaps the T-stat is also restricting the flow a little more as well.
In any event, if you can hold your hand on the cylinder head for more then 3 seconds without burning yourself, then it is probably operating at the right temperature. It never hurts to change the impeller and what most boat owners don't seem to get is an impeller has to be changed when it is working well, since the alternative is to wait until it fails. Unfortuneately, the alternative method gives you about 2 minutes notice, at best, if you notice it at all, however in about 2 minutes you will.
