Yamaha 8hp smother problem?

[LeeAdkison]

My 8 hp, 4 stroke Yamaha has a problem where the engine dies if the cover is attached. The outboard mounted to a sailboat that live in the saltwater. It runs fine without the cover but as I slowly bring the cover down on the engine, it's like snuffing a candle. I recently discovered it also runs fine out of the water with the hose attached and cover on. This motor has a hose attachment point and I tried running it with the hose attached while on the boat in the water. Same problem. Tilting the motor out of the water with the hose attached, it ran a bit rough but did not run down to quit. It's acting as if, when the prop is in the water, the exhaust is not getting out so it's venting into the cover so that, when the cover is attached, the exhaust smothers the engine so it can't breathe. However, apart from a small bit when I first start it for the day, I don't see and smoke around the head of the motor. It will run fine in the water with the cover off and it will run at near top RPMs without dying with the cover on. There are no visible leaks of water or fuel. It's a clean looking engine (2001) and a recent checkup showed it to be otherwise in good shape with good compression. Any suggestions would be greatly appreciated.