Thanks pe reivers!
I forgot to mention the tank had two lines; the one with the weighted end headed straight to the carb (well, I had a gas filter in between), and the other was plugged (used only for refilling). Now I realise that, without the vent line, if I had ran the engine for a long time it would struggle to suck fuel beyond a certain point. [:@]

Instead of making a 3rd hole, can I just unplug the 2nd line and refill throught it when I need it? Also, is there an "elegant" way to make the vent line, so that fuel won't spill when you do certain maneuvers?