I am having a problem with my 1997 25 h.p. Johnson. When I first start it up ot runs great, and as long as I am running it pretty well open the water just pumps out fine, but the problem is when I am trolling and the motor is at idle, these i not water pumping and it gets hot... Why will the water pump at mid-and - ful throtle, but not water at an idle? Any idea, and how do I go about fixing it? Thanks!