I'm still a bit confused, though. Even if the #2 carb were overfilling, wouldn't the needle allow about the same amount of fuel into the combustion chamber?
Nope. The fuel level directly impacts the air-fuel ratio. At small throttle openings, all or nearly all of the fuel flows through the low speed circuit. As the throttle is opened more, the carb transitions from low speed (pilot) operation to the main jet. In both cases, what comes out of the jets in not pure gasoline, it is an air-fuel mix. If the fuel level is too high, then fuel can enter the emulsion passages that are supposed to introduce air into the fuel, hence causing a rich condition. If the fuel level is really high, it can force gasoline through the main jet before there is demand for it, causing an even richer condition.
If the fuel level is too low, it can cause too much air to be introduced into the jets, or even prevent the jets from picking up any gasoline at all.
