Re: 18 johnson//carb adj.???
I think you got a 1962. I have several OMC 18's and the carbs are always in need of tuning. The slow/idle jet adjustment should turn more than half a turn. There is a set screw that holds an indicator in on some models (older). If you undo the screw you might be able to move the jet.
A good baseline to work from is to screw the jet in all the way, being careful not to go too hard or too far or you will flatten the tip. Unscrew 1.5 turns. Then, adjust while the motor is running, moving the screw either clockwise or counterclocwise. Do small movements, since it is very sensitive. Wait a few seconds (around 10) for the motor to respond to the carb adjustment.
You should find a nice comfortable position. Put your set screw back in with the indicator in the middle of the range.
Usually simple as that. I hope this is what you were looking for.
