What Steve said.
The other thing to bear in mind is that, in most carbs, the main jet is separate from the needle jet/emulsion tube. It's screwed in right at the bottom. So the main jet usually only has a direct effect on the a/f ratio at wot (wide open throttle). Maybe just a bit below this, but not much. The reason is that, at throttle position below wot the needle taper and how it sits in the needle jet have control. In other words, until the needle is lifted to wot it's taper provides a smaller surface area for the fuel to get through.
True, a small bike ridden with...er...... Enthusiasm, will spend a lot of time at wot! But for operation at intermediate throttle positions the needle taper and/or height relative to the slide matter. On older slide carb designs the slide had a cutaway, which had control over the a/f ratio at very low throttle positions, but I'm not sure that cv carbs have the facility for this?
As Steve hinted, there is more to a cv carb than perhaps meets the eye. Removing the airbox will almost definitely alter the pressure felt on the lower side of the diaphragm. Whether and to what degree this screws up the a/f ratio seems to vary widely in different types of bike.
Checking the main jet is, I guess, going to be the easiest to eliminate from your worry list, at least on a little bike. Find a situation where you can safely give it full throttle and kill the ignition whilst at that throttle position, as Steve said. If you have your collar felt, remember that Steve advised you to do this.
