Good Article describing SU function - How s30 carbs work

[cockerstar]

While searching for some pics of round top set-ups I stumbled across this article that I hadn't ever seen before. It has a nice little section describing the function of SU carbs that I thought was relatively easy to understand compared to some of the other things I've read. Just thought that it would be good to have for people searching on here so I tried to make the title have a few key words in it. If anyone else has some good information to add, feel free

 

SU carburetors (named for Skinners Union, the company which produced them) were a brand of sidedraft carburetor widely used in British (Austin, Morris, 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 in production through to the 1980s by which time they had become part of the BMC/British Leyland Group. Hitachi also built carburetors based on the SU design which were used on the Datsun 240Z and other Datsun Cars. While these look the same, they are different enough that needles (see below) are the only part that fits both.

SU carburetors 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 carburetor. 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 in communication with atmospheric pressure. The difference in pressure between the two sides of the piston creates a force tending to lift the piston. Counteracting this force are the force of the weight of the piston and the force of a compression spring which is compressed by the piston rising; because the spring is operating over a very small part of its possible range of extension, the spring force approximates to a constant force. 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, 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 carburetors by temporarily increasing the speed of air through the venturi, thus increasing the richness of the mixture.

The beauty of the S.U. lies in its simplicity and lack of multiple jets and ease of adjustment. Adjustment being carried out 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 used on many motorcycles where the main needle position is raised and lowered by a direct connection to the throttle cable rather than indirectly by the depression in the venturi. However, this apparent similarity is misleading. The piston in a motorcycle-type carburettor is controlled by the demands of the rider rather than the demands of the engine, so the metering of the fuel is inaccurate unless the motorcycle is travelling at a constant speed at a constant throttle setting - conditions which are rarely encountered except on motorways. This inaccuracy results in the wasting of fuel, particularly as the carburettor must be set slightly rich to avoid damaging leanness under transient conditions. For this reason Japanese motorcycle manufacturers ceased to fit slide carbs and substituted constant-depression carbs which are essentially a miniature Japanese SU. It is also possible - indeed, easy - to retro-fit SU carbs to a bike which was originally manufactured with a slide carb, and thereby obtain improved fuel economy and more tractable low-speed behaviour.

 

 

 

[Daeron]

I just skimmed it and it looks rather like an abstracted and shortened version of the description they have posted at Ztherapy, the carb rebuilder's website. That site also has the benefit of having a few actual photos interspersed with the article and schematic drawings. Anyone who finds this article enlightening would do well to google Ztherapy and spend an hour or two reading through their website.

 

 

Great little carbs, though, aren't they?

[cockerstar]

They are great carbs

I don't think that I've ever actually read through the whole thing on zTherapy although I have looked at their pics for references multiple times.

One thing that really helped me learn about the SU's workings was to search youtube. There were some videos of automotive school classes that were quite useful.

 

The page I linked to gave a basic explanation of carbs in general and then a little detail on multiple kinds. Good to know a little bit about from a lot of what's out there.

[Guest mattzellhart]

I found this YouTube video was very helpful for the visual people like me.

Pt1:

 

Pt2: