Carburettor Breather?

[Dead Eye]

So I got a rig setup and checked all the float heights after stripping out the guts of each bowl and cleaning them (bit of carb cleaner and some air from my compressor - yes I forced the air in the reverse direction of flow). Each one was within spec so I guess the carbs are going back on and I'll try again.

I'm still a little confused as to how fuel came out of the pipe I highlighted in the original post - it connects above the butterfly and slider and I didn't think the fuel got that high (or was supposed to)