Fuel Tank overflow drain tube.

[Lord Popgun]

There is no gate. The ‘Overflow’ hose is part of the vapor recovery system, if you have it in Maylasia. If you do the hose goes to the evap canister where vapors can be drawn into the engine if the purge valve is energized. There is also a drain hose out of the canister. If fuel is coming out the canister is most likely saturated.

You may not have all this on your bike. In any case it sounds like you are overfilling the tank. There is no way for fuel to reach the overflow tube if properly filled.