What makes a double pumper a double pumper?

As fuel demand increases with performance, the need for more fuel arises. A vacuum secondary carburetor only has one accelerator pump and one squirter on the primary side. A really hot street or race engine will benefit from the immediate discharge of a second pump shot on the secondary side, just as the secondaries begin to open. Carburetors without an accelerator pump on the secondary side rely completely on the siphon effect to supply fuel to the secondary side. As the performance of the engine increases, that is often not enough, so another accelerator pump is added.