This plunger piston is part of the accelerator pump mechanism on Weber DCOE and DCO carburetors. When the throttle is closed, the pump rod pulls the plunger upwards to draw fuel into the pump. Opening the throttle releases the rod, and this piston comes down to shoot fuel through the pump jets.
The plunger should be a close fit in the pump bore, but it mus also move freely. Corrosion or scoring will cause problems in the accelerator pump operation.
Sold individually; one required per carburetor. Does not include pump rod or spring.