Designed and developed by Dennis Barbet (Standard Triumph) and Harry Cartwright (Zenith) to break SU's patents, the Stromberg carburettor features a variable venturi controlled by a piston.

This piston has a long, tapered, conical metering rod (usually referred to as a "needle") that fits inside an orifice ("jet") that admits fuel into the airstream at the venturi of the carburettor.

With appropriate selection of the needle, the fuel delivery can be matched much more closely to the demands of the engine than is possible with the more common fixed-venturi carburettor, an inherently inaccurate device whose design must incorporate many complex fudges to obtain usable accuracy of fuelling.

This self-adjusting nature makes the selection of the maximum venturi diameter (colloquially, but inaccurately, referred to as "choke size") much less critical than with a fixed-venturi carburettor.

To prevent erratic and sudden movements of the piston, it is damped by light oil in a dashpot (under the white plastic cover in the picture), which requires periodic topping-up.