As the plunger of a mechanical injector moves up, what allows fuel to flow into the injector cup?

When the plunger of a mechanical injector moves up, it creates a vacuum or decreased pressure inside the injector cup. This pressure difference between the injector cup and the surrounding fuel supply allows fuel to flow into the injector cup. The opening of the metering orifice permits the fuel to enter as the plunger ascends, thus allowing the fuel to fill the injector cup due to the pressure differential that is established.

Closing the metering orifice would restrict fuel flow, while increasing pressure or lowering temperature would not facilitate the flow of fuel into the injector cup.