It could be a number of things.
It could be the floats, the float spring, a bad float needle or just something stuck at the float needle.
You might be able to dislodge it with a few "gentle" taps on the carb. :woohoo:
The float bowl comes right off by sliding the retainer back.

It may be time to give those carbs a good cleaning.
It's not as hard as you might think... just keep track of where the parts go !

It is as 11190 said. Most usually a small spec of trash caught between the float seat and the float needle. If you work carefully over a white kitchen paper towel, sometimes you can collect the offending trash and see it for yourself. It will usually be about 1/2 the size of a grain of sand. So we're talking really tiny.

To effect an optimal repair one must think in terms of symptoms and causes. Sometimes you have simply gotten a spec of dirt into your fuel system, which has unfortunately decided to take up residence in the float needle seat. (A direct cause.)

However, it could also be a symptom of something larger, say rust forming inside the fuel tank, or debris and scale from older fuel unwisely left in the tank during storage. These 2 items will often start to break loose and enter the carbs once the motorcycle has been placed back in service. If that is the case, then more rust and scale are soon to follow, and cleaning the carbs will become a daily service routine. In these instances, I highly suggest that you add 2 fuel filters, one just below each fuel tap.

Hope this helps.