Originally Posted by Simosez
float valves leaking. replace these.
It is very rare for there to be an issue with a float seat. Usually the float needle and/or the needle seat o-ring are at fault.
Also make sure that the float itself doesn't leak, i.e. that there's no fuel inside the two chambers. You can either hold it up to a light, in which case it should be evident if there's fuel inside, or you can put it in a container of liquid and see if one side sinks more than the other.
suggest a full rebuild of the carbs when you have them out.