Carb Circuits
Hal,
You are seeing it correctly, and lowering the needle will lean the entire range over which the needle jet is operable.
Regarding the main jet, look at the exploded view in the 1st link I sent and find item #36 near the bottom the illustration - this is the main jet. Depending on the particular carb model, the main jet may be mounted vertically in the bottom of the carb stem, or may be mounted horizontally near the bottom of the carb stem. Regardless of which of these positions it is in, the main jet function is identical, i.e., it supplies fuel to the needle jet circuit at partial throttle positions, and is the sole metering device at wide open throttle (WOT). If you view the second link above you can see which circuits are recruited and blended at the various throttle positions.
All fuel fed through the needle jet/jet needle circuit must 1st pass through the main jet, as this is the only fuel supply for the needle jet. The main jet can obviously pass more fuel than is required by the needle circuit, thus it is not metering anything until you are nearly at WOT. At WOT the jet needle is essentially withdrawn from the needle jet and the circumferential area between the jet needle and needle jet becomes so large that the needle circuit becomes inactive, leaving the main jet is the sole metering device.
To provide an example of main jet size influencing needle circuit operation consider the following. A friend of mine was attempting to size a main jet correctly for an experimental engine, running a smooth bore Mikuni, and installed a series of increasingly larger main jets in his side valve engine and test drove it after every change to determine when it was too rich. The jet size varied from a quite small (which was about the correct size) to quite large (dia increased 1.4x over starting point, which is a huge change, and would make any engine pig rich). He could tell no difference at all between any of the jet sizes, finding that the bike ran equally well with all of them. Well, we knew this couldn't be true, yet it actually happened. So what was actually going on here? Well, come to find out after the fact, he looked in his carb one day and found that, due to incorrect twist grip travel, he was only opening the slide about 3/4 of the way, so in fact he never was on the main jet, only the needle circuit. In this case even though the main jet was huge it did not significantly alter the operation of the needle circuit. If he ever would have opened it up with the huge main jet present, it would have fallen flat on it's face, blubbering and stuttering to a standstill.
