The problem with motors on sailboats, is that they hardly ever get used. That allows the fuel to turn to varnish and for critters to build nests in air and water passages.
Always use some sort of fuel treatment (not just a stabilizer)....something like Sea Foam, Pri-g, Stabil-Marine, etc. That should help keep your fuel fresh; help clean carb passages; decarbonize the engine; and counteract the effects of ethanol.
When finished using the motor...run the motor out of fuel...then drain the carburator.
So much for the preaching.
To answer your question, the idle jet screw you are referring to is actually called the idle mixture screw or sometimes called the pilot screw. Opps...I thought I said no more preaching.
I can not tell from the picture whether there is a limiting cap on your screw. I have indicated the location of the idle mixture screw in the photo.
You might get lucky just adjusting it. You probably really need to remove the carb, disassemble and clean the whole thing...but you can try this and you might get lucky. Most likely you have some debris in the idle circuit and it clears sometime, then it plugs.
Drain the carburator...and leave the drain screw open
Remove the idle mixture screw (there is a spring on the screw, so do not lose that. Take a can of brake clean or carb cleaner (I prefer brake clean...since it is less corrosive) and spray a few shots through the straw that comes with it, into the hole where the screw came out of.
If you have access to compressed air or at least if you have a small can of compressed air (that you use to clean your computer keyboard)...sticks its straw into the hole and give it a few shots. Even just blowing on a standard straw by mouth is better than nothing. Hopefully, that will clean out any small debris that may be in your idle circuit.
Then, leaving the drain screw open, pump the fuel bulb and flush the carburator allowing fuel and hopefully any debris to flow out.
Close the drain.
Put the idle mixture screw back in and screw it all the way until it is lightly seated. The back the screw out
2 3/4 turns. If your motor serial number is 1300001 or greater, then turn it out 3 turns.
That is your initial setting.
To adjust it, start the engine and let it warm up. (a few minutes) At idle, put the motor in gear and turn the screw either way about 1/8 of a turn and see if the motor speeds up or slows down. Turn it back to were you started, then turn it the other way and see if the motor speeds up or slows down. You are basically trying to find maximum idle. Generally, the correct position will be almost right on the initial setting if the carb is good and clean.
I have also pointed out the crankcase breather hose. That hose goes through a hose in the lower part of the engine case. Critters like to make nests in there. Pull the hose off of the nipple and blow down through it to make sure it is clear of debris.
Good luck and smooth sailing.
Mike