Yes, your correct, the jets don't have adjustments to them (no locknuts or range of adjustment, just "snuggly tight"). The air mixture screws are under those brass caps that you mentioned. Yes, you may very well need to clean them, too. The caps are there to keep consumers with mucking with them that can affect settings made at the factory to make sure the engines meet emissions standards. They are simple to remove:
1) Remove the carbs from the intake boots (obviously after removing the airbox!).
2) With a small drill bit (about 1/16th" or 1.5mm), carefully drill through the cap--just until the bit goes through! It's not very far. If you let the drill just punch through, then you could damage the airscrew and that's a problem.
3) Take a steel screw and screw it into the hole a couple of turns.
4) With some pliers/Vice-Grips, grab the screw head and work the cap out of the hole.
It's pretty easy to do. No, you don't need to replace the caps...
If you do this, make sure to be careful when removing the airscrew: there's the screw, a spring, a metal washer, and a rubber o-ring (in that order) and make sure they go back in reverse order (o-ring, washer, spring, screw). Also, before you remove them (BEFORE!!!!), carefully screw them all the way IN, counting exactly how many turns it takes on each one (down to the quarter-turn, as in 2 turns, or 2.5, or 2.75, or whatever). Quite often, you'll find they aren't all the same, so be sure to right it down (I usually work from left to right, except when I'm in the Middle East). IF you are happy with the way your bike ran before the winter, then make sure to put them back to the same position (screw them all the way in, then back them out the number of turns you wrote down). If not, I think the factory adjustment in the manual is 2.5 turns out on all cylinders. That's why I use (with a carb jet kit) and I've been very happy with the results. YMMV.