These two accelerator pump plungers are from a Rochester 2G small bore carburetor, the kind frequently favored for tri-power applications. The rubber cups on these pumps are Buna-N synthetic rubber. Buna-N served its purpose well before the days of ethanol, but doesn't hold up well with the alcohol content of modern gas, which can cause the material to swell, and deteriorate. It may not look deteriorated, but look closely at the used cup on the right and notice that it has pulled itself inside-out and no longer fits correctly on the piston. If you could touch it, you'd notice that it is hard and has lost its elasticity.