I concur with Rascott, the problem is almost certainly to be found in the float/float needle and seat assemblies..
In all probability its probably just some crud (usually varnished gas) in the needle seat making the seal unreliable..
Ive fixed a few of these by applying valve lapping compound on the needle, hooking a rubber tube to it, then spinning it in the needle seat with an electric drill, just like lapping a valve, but easier as the brass is softer. Its an old Mikuni trick and gives perfect results if you see a nice shiny new seating area on both parts. With the multi carbed Jap bikes a set of 4 needle and seats go for about $150 bucks, which gives incentive to just regrind them vs replace them.. Ive repaired hundres of motorcycle carbs this way, and a few of the carbed Samuris too.
Make deadly certain too remove ALL off the lapping compound..