S. U. Excellence - working on the wonderful Skinners Union Carburettors and hello again to Harry.
The most under-rated carburettor.
The basic idea was patented by Herbert Skinner in 1906 but the typical S.U. carb’ we know today was developed over the following 80 years until the almost universal adoption of fuel-injection consigned the carburettor to history. The last S.U.s for a car maker were made for the Rover K-series engine in late 1980s. After several changes of ownership, the S.U. brand and manufacturing rights were brought back to the UK by Burlen Ltd. For more history, technical specification and loads of diagrams go to http://sucarb.co.uk/
Twin S.U. HS4 carburettors fitted to the Nagara MG Midget 1500 Continental with a new custom air-box under construction a few years ago.
I’ve worked on S.U. carbs for over 50 years but I’ve only really got to know them reasonably well comparatively recently. They were used on a huge number of cars, motorcycles, boat engines and other petrol engines all over the world. They distinguish themselves because they deliver a fuel-air mixture to the engine in a most ingenious way: they were designed to supply a variable amount of fuel and a variable amount of air to suit the requirements of the engine before the invention of computers or electronic assistance. That such subtle metering of the air-to-fuel ratio is done by purely mechanical means is both extraordinarily clever as well as aesthetically pleasing.
The variable fuel supply is via a tapered needle that moves in a hole (the jet) so that when the needle is raised, more fuel can exit from the jet and be sucked into the engine. The air is metered thusly; the needle is attached to a small piston that, when the needle is down, almost entirely blocks the passage of air into the engine via a tube known as the ‘throat’ or ‘choke’ of the carburettor. The piston itself has its top part slid into in a chamber called the ‘dashpot’ open to the air at the bottom and sealed at the top except for passage ways to the ‘induction’ side of the carb’s throat. The throat itself is opened and closed by a simple butterfly valve called the ‘throttle’. Because the engine behaves like a pump, the air pressure on the induction (inlet) side of the carb is lower than atmospheric pressure. When the throttle opens, atmospheric pressure pushes the carb’s piston up, so opening the air-way. There are lots of technical diagrams on the Burlen site and here’s a beautifully simplified version that shows a section (as if cut in half) of a typical S.U. carb and how it works.
For those unfamiliar with looking at section diagrams, imagine cutting an apple in half and looking at the cut surface, the skin would be a thin line around the apple. If an orange, the pith and skin would be a thicker line. In the above diagram the same rules apply with the shaded areas being ‘solid’ material and the dotted lines being items of interest that are not in the plane of the cut. Small items are just line drawings as they are too small to shade easily.
Another nice feature is the damper - basically a ‘shock absorber’ (a bad name for something that is a damper!) for the piston; it stops the piston from moving up and down too readily if there is a slight change in the air pressure either side of the piton so that it operates smoothly.
As time went on, and the requirements of motoring became more demanding in the 1960s, the S.U. became more refined and a bit more complicated. The basic design principle was the same but additional features such as an accelerator pump were added - combining the advantages of a fixed jet carb with the variable jet design. Such a carburettor the SU HIF6, two of them in fact, feed fuel into the wonderful Turner designed V8 engine in the rare and sensational 4.5 litres Daimler Majestic Major. The Daimler is pictured below with me and owner Harry Edwards.
Harry and I first met in the early 1980s but lost touch about 10 years later only to be in touch again just a couple of months ago. It is partly, well, mostly, my fault that Harry bought the Daimler in the first place - but that is a whole other story. Over the years Harry has had the car restored and fettled but it has rarely been driven. It was time to recommission it with a good service and a check-over.
On a trip to London I called in and over cups of tea we reminisced and chatted while I took the carburettors off the car to sort out leaks and poor operation. They went into a sturdy bag with me back on the train to Devon. Once in the workshop, they were dismantled and cleaned, fitted with new float-chamber needles and seats (not shown in above diagram and entirely different from the long needles that operate in the jets) and reassembled with new gaskets.
If your classic car has an S.U. carb and you’re not sure what type it is, there’s hopefully a small tag attached to one of the screws holding the float chamber lid with the carb’s number stamped on it. If you give Burlen a ring, they can probably tell you which carb the reference number belongs to.
Take numerous pictures of the linkages and the way they are oriented, it’s very helpful when it comes to reassembly. If all seems well, and the links operate smoothly with no slop then leave them alone and just clean them thoroughly.
After an initial clean, the first carb is dismantled.
Here’s the body with all major parts removed.
The piston should be free to slide easily in its dashpot. Both of the Daimler’s carbs were stuck - one stuck down and one stuck up due to corrosion between the piston the side of the bore in the dashpot.
The gasket between the mouth of the carb and the lovely curved cast aluminium velocity stacks/intake tubes were not in the rebuild kit for the floats so I made two gaskets from gasket paper.
One of the carbs is nearly finished
Cleaned parts set aside in a box awaiting assembly - note the plastic box (old washing sachet box) with a lid for small parts that otherwise will inevitably get lost when your tired and knock something tiny off the bench and into the unknown.
The two carbs all ready to go back to London and be fitted on to the Daimler.
Racers love Weber carburettors and, at full throttle, they do offer some advantages. But, for the road, an S.U. is the better choice; it comes on and off throttle so instantly and smoothly, it is easy to tune and service and, to me at least, the S.U. has a sculptural quality that goes beyond mere performance and racers can win with them too. The famous American architects Louis Sullivan and Frank Lloyd Wright coined the phrase ‘form follows function’ and in nature we see that kind of beauty all the time. While there is little that is not functional on an S.U. Carburettor, there is much that is detailed in such a way to look beautiful - after all, engineers are part of nature too.
My thanks to Harry Edwards.