merc 3.5 question

[kenfyoozed]

I have a 1985 merc 3.5hp outboard. The water pump seemed to stop working the last time out. So i checked it and it was clogged with debris, i cleanned it out, and cleaned up the inside of the leg. While doing so i noticed that at the top of the leg , where the water is pumped to cool the motor, there is a large screw on the back side. I took this screw out and water pours out while its running. Whith the screw in, the water that cools the leg is hotter than if the screw is out. When its out, the water just runs out the top of the leg. It is also pumping water out the smaller holes on the leg, but my question is what is that screw for? and should it be in? It seems like it would cool better if it wasnt.<br />Thoughts?