Re: 1998 50 HP Johnson carburator
To do it right you will need to get the boat and motor in the water (not just at home on muffs or in a bucket), get the engine warmed up, then set it while idling in neutral. I have a 2000 55hp commercial motor, very similar to yours, and I had to set the idle adjustment screws on the sides of the carbs like 4.5 turns out just to get it so it would run good enough to set it. The high speed I believe is a fixed jet, at least mine is.
If you have a repair manual it should cover this information.