SU Carburettor Explained
SU carburettors (named for Skinners Union, the company that produced them) were a brand of carburettor of the sidedraught constant depression type. A handful of downdraught variants were used on some pre-war cars.
They were widely used in British (Austin, Morris, Jaguar, Triumph, MG) and Swedish (Volvo, Saab 99) automobiles for much of the twentieth century. Originally designed and patented by George Herbert Skinner in 1905, they remained on production cars through to 1993 in the Mini and the Maestro by which time they had become part of the Rover Group. They are now manufactured by Burlen Fuel Systems Limited mainly for the classic car market. Hitachi also built carburettors based on the SU design which were used on the Datsun 240Z, Datsun 260Z and other Datsun Cars. While these appear the same, they differ to the extent that needles (see below) are the only part that fits both.
SU carburettors featured a variable venturi controlled by a piston. 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. Since the needle is tapered, as it rises and falls it opens and closes the opening in the jet, regulating the passage of fuel, so the movement of the piston controls the amount of fuel delivered, depending on engine demand.
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 lifts 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. To prevent erratic and sudden movements of the piston it is damped by light oil in a dashpot, which requires periodic replenishment. The damping 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 an 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 an SU carburettor. This piston actuation difference is the significant distinction between a slide and an SU carburettor. The piston in a slide carburettor is controlled by the operator's 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 rarely encountered except on motorways. 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). For this reason Japanese motorcycle manufacturers ceased to fit slide carbs and substituted constant-depression carbs, which are essentially miniature SUs. It is also possible - indeed, easy - to retrofit an SU carburettor to a bike that was originally manufactured with a slide carburettor, and obtain improved fuel economy and more tractable low-speed behaviour.
One of the downsides 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.
Maintenance of the SU Carburetor
Presuming the ignition system works correctly and the engine passes its compression test, you should consider the carburetors.
First of all, run the engine and determine that the throttle spindles are not leaking air inside the carburetor bodies. These spindles pass laterally through each carburetor, to which the butterfly throttle valve is attached. If the valve and/or spindle are allowing in air, the carburetors cannot be adjusted properly.
Since the engine is running, a vacuum is being created inside the carburetor. Drip some oil where the spindles enter the carburetors, to see if the vacuum draws oil inside. If this happens, the rods and their bushings need replacement. Some cars have gaskets at these joints, to prevent air intrusion.
Check the exterior of the carburetor for indications of any leaks. Very slight leaks appear as dark, gummy residue left behind when gasoline evaporates, leaving the additives behind. Larger leaks appear as clean streaks in such gummy residue, caused by liquid gasoline. Especially, check the lid of the float bowl, the gaskets between the float bowl and the carburetor, the 'Banjo' fittings where fuel lines attach to the float bowls and the bottom of the carburetor body where gasoline might leak around the needle and jet. Obviously, replace any leaking gaskets.
As in nearly all carburetors, the level of fuel in the float bowl is critical to proper engine operation, because gravity feeds fuel from the bowl to the jet inside the carburetor. If the fuel level is too high, then there is too much fuel at the jet and the engine runs too rich. If the level is too low, the resulting lean mixture burns too hot and tends to burn the exhaust valves. Thus, the first thing to do is to ensure the float bowl fuel level is correctly set. Only then can other adjustments be undertaken with accuracy.
The float inside the float bowl should be checked for leaks, because a heavy float rests lower in its pool of gasoline, allowing extra fuel to enter the chamber and raising the overall fuel level in the bowl. Remove the float bowl's top. Remove the float with a straightened paperclip that has a 1/4" right-angle bend at the end. Shake the float, to see if gasoline appears to be inside. Replace the float if necessary. There is no reasonable way to remove the liquid and seal the float. Floats are inexpensive.
As with most carburetors, a 'needle and seat' in the float bowl modulates the fuel flow. When the engine burns fuel, the level drops in the float bowl. A forked lever mechanism opens the 'needle and seat' arrangement in the float bowl's top, to allow more fuel into the bowl.
To adjust the float bowl, lay a sized rod (given in the workshop manual for the particular model car) between the forked lever and the top of the float. The forked lever should barely touch the rod when the 'needle and seat' valve is closed. To adjust, bend the forked lever where the forked arms begin.
Check that when the 'needle and seat' is closed, no air can get past it. If air can pass, replace the 'needle and seat.' This air check means removing the 'Banjo' fitting from the float bowl top, an operation best performed while the top is bolted on the float bowl. After replacing the 'needle and seat,' perform another air (or vacuum) check. On rare occasions, new needles and seats can be faulty.
A leaky 'needle and seat' in the float bowl is the primary cause of burned exhaust valves. The reason is that the SU carburetor mixture is adjusted at idle speed. If the 'needle and seat' assembly is leaking, then a constant flow of excess gasoline enters the float chamber.
This artificially raises the level of gasoline into the float, which makes the carburetor run rich, but this is at idle only. At speed, the small amount of excess gasoline is inconsequential.
When the carburetor mixture is adjusted correctly at idle speed, allowing for the extra gasoline, the carburetor becomes too lean at normal operating speeds. This means that the engine's combustion chamber runs too hot at speed and the exhaust valves burn.
Over the years, the manufacturer has recognized this problem and has developed needles with longer lives, such as needles with neoprene tips. Since the 'needle and seat' is a inexpensive unit, you should replace them all with each tuneup.
Tiny, brass fuel filters fit between the 'Banjo' fittings and the float tops. These can become clogged with a clear substance. They can be hard to find. You can replace them or you could even put one on a steel surface, place one drop of gasoline on it and set it on fire. The deposits will burn off and the brass filter can be reused. Replacement is probably best.
Re-install the top onto the float bowl before final tightening of the 'Banjo' fuel line fitting, to prevent undue stress on the fuel lines.
When you remove the carburetor's bell assembly, pressurize the fuel system by turning on the ignition to operate the fuel pump. Some cars, like the Triumph have mechanical fuel pumps which can be primed by hand via a lever underneath.
Remove the carburetor 'bell' and its piston. Look inside the carburetor's jet. Fuel should be visible about 1/32th of an inch below the opening, indicating that the float bowl is performing properly. No fuel may be running past the jet, or else the float bowl must be fixed. If there is no fuel visible, the fuel level is too low.
Check the sides of the long, tapered needle for scrape marks. If you find them, you must replace the needle AND the jet because the jet will be elongated, making the engine run rich at speed.
If you replace the jet, you will need to "Centralize" it so that it does not scrape the needle. The carburetor is constructed so that it can slide around sideways. The large nut (Whitworth size) holding the jet in place gets tightened after you have ensured the jet does not impinge on the tapered needle, whatsoever.
I have found that centralizing the jet is best performed on the workbench.
With the bell back on and the carburetor on the engine, fill the oil chamber with light oil. Motor oil is okay because the viscosity of the oil is not critical.
SU Carburetors are often installed two or three per engine and their airflows must be synchronized. If one carburetor is consuming most of the air at idle, you will not be able to adjust the other carburetors. While manuals say you can synchronize air flow by using a rubber tube in the intake, with the other end in your ear, this method is too approximate. A carburetor synchronization tool should be used.
With the carburetors consuming equal amounts of air at idle, and the air filters off, lift a piston with a thin screwdriver. This action artificially leans the mixture on that carburetor. The engine should speed up momentarily and then drop to a slower speed. If you don't see the speedup, the carburetor is running too lean. If you don't see the subsequent slowdown, the carburetor is running too rich. If the slowdown settles to a loping misfire condition.
Some carburetors have small buttons that allow you to lift the pistons without removing the air cleaners, so you can adjust the mixtures.
Each carburetor should act the same. If this doesn't happen, then the air synchronization is inaccurate.
Check the carburetor linkages to see that the butterfly valves visible inside the throats open together. With the engine turned off, block the throttle about 1/2 way open and check that the butterfly valves are equally open. If they are not, then adjust the throttle linkages.
Renew the air filters. Some cars have filters of metal mesh. Apply some motor oil onto these, after cleaning them in solvent.
The SU carburetor has an undeserved reputation for needing constant care and adjustment. By ensuring the 'needle and seat' assembly does not leak and the 'needle and jet' assembly inside the carburetor is properly centralized, the carburetor performs nicely between tuneups. Set improperly, however, they can cause burned exhaust valves or fouled spark plugs.
SU carburettor types
SU carburettors were supplied in several throat sizes in both Imperial (inch) and metric (millimeter) measurement.
The carburettor 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).
- "HV", "OM" and "KIF" types also exist but were less commonly employed.
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 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 is the throat size in mm. For example, if the serial number is HIF38, the final number is 38, size is 38 mm etc.