We spend most of our mid throttle life (day to day riding) using the needle jets and they wear, along with their bosom buddies the needles themselves. The petrol is inducted up out of the bowl and through the concentric gap between the needles and the needle jets where it atomises (I love it SO much when I use complicated scientific terms in a discussion). When they wear, they do so unevenly and the fuel does not atomise as well and it does not burn as efficiently.
My Aussie flag bike (R100RT)was running a bit lumpily and not getting the full 360 km out of a 24l tank of petrol, so I suspected that they were the culprit. Constant vacuum carbs are a cinch to open up. The needles are situated within the vacuum tubes and are withdrawn by doing a sort of a combination of a twist and a pull. You want to put the new needle back so that it's at its second lowest of the four positions. You need to compare the length of needle sticking out on the opposing carb, so that they are balanced. The needle jet sits on top of the main jet and is simply held in place by the base of the main jet. I rode my bike right away and felt the difference immediately, much smoother idling. After I got my 16km/litre result on the refuel, I chucked the old needles away.
Brian at Munichs will gladly supply you with new parts for a couple of dollars. If you don't patronise the joint, he'll just spend the morning daydreaming, so go along and stir him up.
John
