The flow of air through the venturi creates a reduced static pressure in the venturi. This pressure drop is communicated to the upper side of the piston via an air passage. The underside of the piston is open to atmospheric pressure. The difference in pressure between the two sides of the piston tends to lift the piston. Opposing this are the weight of the piston and the force of a spring that is compressed by the piston rising. Because the spring is operating over a very small part of its possible range of extension, its force is approximately constant. Under steady state conditions the upwards and downwards forces on the piston are equal and opposite, and the piston does not move.
If the airflow into the engine is increased - by opening the throttle plate (usually referred to as the "butterfly"), or by allowing the engine revs to rise with the throttle plate at a constant setting - the pressure drop in the venturi increases, the pressure above the piston falls, and the piston is sucked upwards, increasing the size of the venturi, until the pressure drop in the venturi returns to its nominal level. Similarly if the airflow into the engine is reduced, the piston will fall. 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 speed of the airflow.
Since the position of the piston controls the position of the needle in the jet and thus the open area of the jet, while the depression in the venturi sucking fuel out of the jet remains constant, the rate of fuel delivery is always a definite function of the rate of air delivery. The precise nature of the function is determined by the profile of the needle. 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. The well-controlled conditions under which the jet is operating also make it possible to obtain good and consistent atomisation of the fuel under all operating conditions.
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. A two-inch SU carburettor is a useful device to have in the workshop when experimenting with engines, as it is possible to bolt it onto more or less any engine and the engine, if in good order, will burst into life without the need for complex carburettor adjustments to get it to start.
To prevent erratic and sudden movements of the piston it is damped by light oil in a dashpot, which requires periodic topping up. The dampening is asymmetrical; it heavily resists upwards movement of the piston. This serves as the equivalent of an "accelerator pump" on traditional carburettors by temporarily increasing the speed of air through the venturi, thus increasing the richness of the mixture.
The beauty of the SU lies in its simplicity and lack of multiple jets and ease of adjustment. Adjustment is accomplished by altering the starting position of the jet relative to the needle on a fine screw. At first sight, the principle appears to bear a similarity to that of the slide carburettor, which was previously used on many motorcycles. The slide carburettor has the same piston and main needle as a SU carburettor, however the piston/needle position is directly actuated by a physical connection to the throttle cable rather than indirectly by venturi airflow as with a SU carburettor. This piston actuation difference is the significant distinction between a slide and a SU carburettor. The piston in a slide carburettor is controlled by the operators demands rather than the demands of the engine. This means that the metering of the fuel can be inaccurate unless the vehicle is travelling at a constant speed at a constant throttle setting - conditions which are rarely encountered except on motorways. This inaccuracy results in the wastage of fuel, particularly as the carburettor must be set slightly rich to avoid a lean condition, which when performed repeatedly can cause significant engine damage. For this reason Japanese motorcycle manufacturers ceased to fit slide carbs and substituted constant-depression carbs which are essentially miniature Japanese SUs. It is also possible - indeed, easy - to retro-fit an SU carburettor to a bike that was originally manufactured with a slide carburettor, and thereby obtain improved fuel economy and more tractable low-speed behaviour.
The only real downside of the constant depression carburettor is in high performance applications. Since it relies on restricting air flow in order to produce enrichment during acceleration, the throttle response lacks punch. By contrast, the fixed choke design adds extra fuel under these conditions using its accelerator pump.
SU carburetors were supplied in several throat sizes in both Imperial (inch) and metric (millimeter) measurement.
The carburetor identification is made by letter prefix which indicates the float type:
"H": in which the float bowl has an arm cast into its base, which mounts to the bottom of the carburetor with a hollow bolt or banjo fitting. Fuel passes through the arm into the carburetor body. The bolt attaches to the carburetor body just behind the main jet assembly.
"HD": the float bowl mounts with its arm fastening directly below, and concentric with, the main jet. The arm has a flange that fastens with 4 screws to the bottom of the carburetor, and sealed with a rubber diaphragm integral with the main jet.
"HS": the float bowl is rigidly mounted to the carburetor body, but fuel is transferred by a separate external flexible line.
"HIF": the float bowl is horizontal and integral (hence the name).
The Imperial sizes include 1-1/8", 1-1/4", 1-1/2", 1-3/4", 1-7/8", and 2", although not every type (H, HD, HS, HIF) was offered in every size.
There were also H models made in 2-1/4" and 2-1/2", now obsolete. Special purpose-built carburetors (Norman) were made as large as 3".
To determine the throat size from the serial number: If the final number (after one, two or three letters, beginning with H) has 1 digit, multiply this number by 1/8", then add add 1". For example, if the serial number is HS6, the final number is 6: 6/8 = 3/4", add 1, total is 1-3/4", etc.
If the final number has 2 digits, it's the throat size in mm. For example, if the serial number is HIF38, the final number is 38, size is 38mm etc.