ORIGINAL: DarZeelon
The carburettors that you talked about effectively enrich the mixture during acceleration, since the piston, to which the metering needle is connected, initially rises higher than its running position, so exposing an effectively larger fuel orifice to the airflow.
This has the same effect as an accelerator pump, without the presence of a pump.
Oh rubbish Dar...that's completely backwards. Both the SU and Stromberg CD (Constant Depression) carbs use a hydraulic damper to delay the rise in the piston when the throttle is opened. Although it restricts the rise of the needle and therefore the area of the fuel jet, the DEPRESSION over the fuel jet is greatly increased by the rise in airflow and THIS is what draws through the extra fuel needed for acceleration. Changing the viscosity of the oil in the damper alters the delay in the rise of the piston and is used to vary the amount of enrichment.