Motor Problem(Help Needed)

[BIGJ]

I have an old 1975 25 HP Johnson Outboard that is not running good.
It will run when it is idled up but at idle speed it has a bad miss and will kill.
I have cleaned the carb and diaphram, new plugs, new points and condenser. Changed the fuel lines, new tank and hose and bulb.
It has a lean/rich knob on the front .

Here is a picture of the lean/rich knob and a leak where the motor splits in half.

How do you set the lean/rich knob.(Tighten all the way and back out how many turns)?
Will the leak in this area cause the motor to run bad? Does it have a gasket in this area? Is it hard to fix?