Re: 1997 15 hp evinrude 4 strok
Regarding the prop, just looking like it is too small in diameter is not necessarily going to lead to a problem. You mentioned that it runs 25 - 30 mph. Have you verified this via GPS? It sounds like the prop is not an issue if it runs that fast. If anything, having too large a prop can overheat a motor. In other words too great a pitch and / or diameter for the load the motor is pushing will prevent the motor from attaining its proper full throttle RPMs. That condition is known as lugging. Is your motor obviously screaming at WOT (Wide Open Throttle), or does it feel lazy and sound like it should be revving higher?
Now, you mention that idle is all you get when the motor heats up. As soon as that happens, remove the engine cover and see if you can place your hand on the block. There shouldn't be any hot spots preventing you from keeping your hand on it. If there are hot spots, then there is a problem with your cooling system. I must ask, when was the impeller changed on this motor? If it hasn't been changed in a while, it should be as a critical part of normal maintenance. If the impeller vanes disintegrate, the bits of rubber from it can clog water passages in the block, which can show up as hot spots in your engine. From what you describe it almost sounds like this is a possibility. In that case you will needed to clear out all water passages and water tubes (very important), replace the impeller (very important), and verify correct thermostat operation. A compression check is also a good idea, as too much variance between the 2 cylinders indicates problems (head gaskets, worn or stuck rings, or possible overheating damage, etc.) Being a 4-stroke it is also wise to verify valve clearance to ensure proper sealing and prevent burnt valves.
Anyway, there are a few suggestions. It is kind of late in the season to get out in a boat for this, at least around here. Good luck and let us know what happens.
