Carburetor passage plug made of Viton rubber. Viton is the best material available for carburetor parts because it will not swell after contacting fuel. This little plug goes on top of the hole between the main jet and the pilot jet. Once the stock plugs harden they don't do their job anymore. It's a strange system, but since it's Honda, they must have known what they were doing.
Height: 8.8mm
Diameter at top: 8mm
Diameter at bottom: 5.9mm
Sold as a set of 4.
Honda Part Numbers (Part of the Gasket Set): 16010-425-671, 16010-469-004, 16010-425-305

Fits Honda:
CB750C Custom - 1980-1981
CB750F Super Sport - 1979-1981
CB750K - 1979-1982
CB750L Limited Edition - 1979
CB750SC Nighthawk - 1982-1983
CB900C Custom - 1980-1982
CB900F - 1981-1982
CB1000C Custom - 1983
CBX - 1979-1982
CB1100F Super Sport - 1983