Sorry but I have to disagree. The fuel tank is pressurized from the exhaust line, and once you crack that lid open it looses all it's pressure witch pushes the fuel to the carb. So in all reality it goes lean. Open your lind and hold it there, your motor will lean out.

Deadman is correct, when the tank is opened the pressure is down, and when you fill it full on a darn near empty tank you force a good amount mote fuel in the carb. A simple fix is to make sure you pit with atleast a 1/8 to 1/4 tank left. This way the tank will always have a good pressure and no pushing fuel threw on refills. If you have to pit under 1/8 tank for some resin keep above idle like deadman stated.