The overflow WILL go to your tank vent line (usually connected to the muffler). The overflow will be just like any other fuel tank situation.
I have used the Du-Bro version of the fueling valve for decades without ever a problem.
Keeping a nice scale cowl is to be admired when it makes a killer presentation on the flight line. No need to gouge unsightly holes into and out of cowls.
Someone suggested "dirt" as a problem, but dirt is a problem with or without a fueling valve. Keep your refueling rig sanitary... simple enough?
Run an in-line filter between your fueling valve and the carb (do this with or without a refueling valve).
If the case of the overflow becomes a problem (unwanted splash onto the ground, filling the muffler leading to a flooded engine in certain mounted configurations, or just plain wasting fuel), you can always use a second refueling valve to catch the vent side overflow (into a fuel tank).
ORIGINAL: TPL33
Bought a Great Planes Remote Fuel Fill Valve for a model I'm building. I see there are connections from the fuel tank, then from the valve to the carburator. My question is, what happens when the fuel tank is full, and, where does the overflow fuel go/exit?