Damper caps are fitted to earlier mainly pre war Carburettors where heavy or weighted Pistons with no Damper are fitted. Some have vents and some do not and they should not be fitted incorrectly. Dampers are designed to Slow the rise of the Suction Piston in the Carburettor in order to give a richer mixture on snap acceleration. They vary considerably from type to type by varying the clearances in the assembled Components. The oil in the Dashpot is forced through the Assembly and thus Slows (dampens) the Piston rise. A further difference is that some are externally vented (hole in the top) and some are internally vented (angled hole in the Suction Chamber) these are termed dustproof and non-dustproof respectively. It is essential that the correct part is fitted to ensure smooth Running of the engine.