Well first you need to replace the fuel selector. I know you cleaned it, but the solvents in carb cleaner actually tend to dry out the seals and make them worse. They’re cheap, so just replace it now...
Second, replace the o ring on your fuel selector. It’s a common place for air leaks, and it’s very important that it seals correctly.
Third, take the carbs back apart and rebuild them with genuine Mikuni parts. DO NOT USE AFTERMARKET CARB PARTS!!! That emphasis is intentional. You will have problems with anything else... There is a carb rebuild sticky on the top of the forum. Follow those instructions exactly, as the carbs are very finicky on these things, and missing something small makes a big difference...