I use these Y pieces on fuel systems where the carburetor does not block fuel flow. The y shape helps force the fuel into the tank with less resistance. You may still need to clamp off the fuel line to the carburetor while filling.
Simply insert one Y piece between the fuel tank outlet and the carburetor inlet, Y end towards the carburetor. Join on a length of fuel tube as a tank filler. Sweet!
The Polypropylene Y Pieces are medical grade so they are guaranteed to keep your models fuel delivery in a healthy condition.