SU carburettor

[1] Herbert Skinner (1872–1931), pioneer motorist and an active participant in the development of the petrol engine,[2] invented his Union carburettor in 1904.

[3] His much younger brother, Carl (Thomas Carlisle) Skinner (1882–1958), also a motoring enthusiast, had joined the Farman Automobile Co in London in 1899.

[7] Later, Carl sold his interest in footwear business Lilley & Skinner and became a partner in G Wailes & Co of Euston Road, London, manufacturers of their carburettor.

[4] Herbert continued to develop and patent improvements through to the 1920s, including the replacement of the leather bellows by a brass piston, even though he was a full-time director and divisional manager of Lilley & Skinner.

With peace in 1918, production resumed, but sales remained slow and the company was not profitable, so Carl Skinner approached his customer, W. R. Morris, and managed to sell him the business.

Carburetter Company Limited,[19] which continues to manufacture carburettors, pumps and components, mainly for the classic car market.

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

The result is that the pressure drop in the venturi remains the same regardless of the speed of the airflow - hence the name "constant depression" for carburettors operating on this principle - but the piston rises and falls according to the rate of air delivery.

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 (20W Grade) in a dashpot, which requires periodic replenishment.

Instead a mechanism lowers the jet assembly, which has the same effect as the needle rising in normal operation - increasing the supply of fuel so that the carburettor will deliver an enriched mixture at all engine speeds and throttle positions.

Adjustment is accomplished by altering the starting position of the jet relative to the needle on a fine screw (26TPI for most pre-HIF versions).

This inaccuracy results in fuel waste, particularly as the carburettor must be set slightly rich to avoid a lean condition (which can cause engine damage).

Since it relies on restricting air flow in order to produce enrichment during acceleration, the throttle response lacks punch.

also produced carburettors for aircraft engines including the early versions of the Rolls-Royce Merlin, but these were of the conventional fixed-jet updraught type rather than the firm's patented constant-depression design.

carburettors were also a popular upfit for Harley-Davidson motorcycles, given their space saving "side draft" design and superior ability to self-compensate for changes in air density/altitude.