When the vehicle passes to fuel from gas, if the switch is brought to the central position then the vehicle   activates both the fuel and gas system. The purpose here is to avoid the engine shake resulting from the full leveled reservoir of carburetor, when it is recognized that the carburetor is full then the switch is brought to the fuel position. This process is only realized when the control is brought to the central position during the transition from gas to fuel.

Both fuels are in interrupted position when you pass from fuel to the central position. In this way, you will be able to discharge the carburetor reservoir.