Hi Koen,
It's only the right one that leaks , other 3 are fine.
So hold the carbs upside down, until the float hits the needle you mean?
In that case, it's quite possible that you have a crack in the overflow standpipe in the float bowl, assuming the float height is not way off. I'm dealing with that very issue right now on my H2. I am in the process of teaching myself how to repair it with either solder or a sleeve, will share what I learn once I have a grip on it.
Hold your finger over the end of the overflow tube and blow compressed air into the drain and see if that's the case. Put your safety glasses on first though :-). In my case the crack was small enough to be hard to see with the naked (53 year old) eye, but the compressed air laid it bare.
You can also test this very simply by simply removing the float bowl and filling it with gas, and see if it leaks holding it in your hand. For some strange reason mine would NOT show a leak when I tested it this way with water, it seems to need gas or similar to flow through that crack, go figure.
Let us know what you find please, there's always something to be learned.