Welcome to the forum! The members here are great and should be able to help you fix your problem. I will just tag-along and try to identify anything that might help my son with his 1995, 4.3L Mercruiser I/O. Good luck!
This is really important to try before you start tearing into the carb! These old beauties run off the LOW speed jet until about 75% throttle movement. If yours is idling well, that's a good indication.