You want to cap off the bottom tube to prevent any syphoning while the model is just sitting, and whn the engine is running, the prop blast over the tube creates a negative pressure that can affect things inside the tank. Similar to chimney effect. A quick and simple fix is to cap the over flow, take a short length of fuel line and cut a 45degree bevel on one end and put that on the fill/uniflow line. This will catch prop blast and air from forward flight and make some positive pressure in the tank. A better way is to install two tubes through the fuselage with the rear one straight, and a length of fuel line going from it to the over flow. This gets capped off. the other will get a 90 degree bend and one end points forward to pick up air from the prop blast or forward motion. Another length of fuel line goes from the other end to the fill tube.  With a standard vented tank, you will get some speed up towards the end of the tank. This set up, or the addition of muffler pressure, helps with that a bit. In the good ld days, some guys with heavy airplanes actually liked this as it helped get them through all the vertical and overhead maneuvers plus the clover leaf.
  Type at you later,
   Dan McEntee