Hmmm...then, this sounds suspiciously like low float bowl levels. If the fuel level is about 2mm low, the idle jets' wells are not sitting in deep-enough fuel levels, and the engine speed has to pick up a little bit to make them able to get enough fuel to sustain low-speed idle. Another thing to look for: make sure the bowl vent hoses are not kinked or blocked (or even missing altogether, but this causes troubles out on the road, not when sitting still) and the bowl vents are all clear and clean.
When you [re]check the floats, look at the little float tang (arm) where the shutoff valve gets pressed closed. Look for a tiny dimple right there. The brass is very soft, and the stainless 18-8 float valve plunger is hard and abrasive (i.e., not polished), so they wear a little dimple. This makes them tend to drive the float level first too deep, then when it starts to refill, it sticks at a slight angle and holds the valve shut, making it fill too low. This process repeats back & forth. I have sometimes diagnosed it by tapping smartly on the the offending carb bowls with a small hammer and wood dowel: if it straightens things up for a minute or so and then the problem returns, the floats are 'sticking', so to speak. File and polish off those little dimples, if this is happening to yours.
On the CB750 carbs prior to the PD series, this is a malady that happens to 100% of them after about 10k miles. On the PD carbs, it takes a little longer. On the 500/550 carbs, it takes about 7000 miles to form the dimples, because the float valve tips are smaller yet.
