The "B" is the emission-controlled version of the "A", which was the emission-controlled version of the earlier carbs.
You can use "A" parts in the "B" version: in fact, if you jet the "B" the same as the "A", and give the "B" a little longer vent-bowl hose to a quieter area (like under the seat), it will work the same as the "A". If you have "B" carbs and suffer from chronic cold-bloodedness, increasing the [press-in] pilot jet from the normal #35 size to the #37.5/#38 /#40 of the "A" will solve it. The air screw setting on the "B" carb is miserly, between 1.5 and 1.75 turns: past 2.0 turns it makes no further difference (richer) because the air port is very small. At 1.75 turns the A/F mixture is about the same as the "A" carb with #38 pilot jet and 1.5 turns, a bit lean.
In both versions: the #1 trouble I find is with the tiny O-rings in the pilot port (air screw seat) having been installed backward by someone (after Keihin). The O-ring goes in first, then the washer. If backward, it will struggle to idle at all.
