Autopsy: SU Carburettor
An elegant piece of design bolted to millions of British classic cars
A closer look inside the SU carburettor and how it works.
Apetrol engine requires a fuel-air mixture of ideally 14.7 parts of air to one part petrol, by weight. It also needs the ability to vary the mixture when required to suit all situations – from start-up, to tickover, to full-load. The SU carburetter, patented in 1906, achieves this with elegant simplicity.
It’s a variable-venturi constant-depression design with a moveable jet. It has a removable fuel-metering needle with a taper tailor-made to provide the correct fuelling characteristics for each application. It’s therefore hugely adaptable, which is why it was used on British cars from AC to Wolseley. The svelte H2 model here ably demonstrates what it’s all about.
[A] BODY The body provides a stable platform for the other components. The alloy die-casting incorporates the bridge, a raised bar across the bottom that houses the jet. We can see it here through the larger round orifice, which is where the vacuum chamber attaches. The small tapped hole about halfway up is for a vacuum advance pipe that goes to the distributor, advancing the ignition timing when manifold depression is high. The symmetrical design of the body allows the choke, float-bowl and accelerator linkage to be mounted on either side.
[B] THROTTLE SPINDLE
This is a 5/16in brass rod with a slit to accept the throttle disc, or 'butterfly'. Two ⅛in BSW screws retain the throttle disc and have bifurcated ends that are splayed after tightening to stop them coming loose. The tolerance between the throttle spindle and the brass bushes cast into the body is so fine that it's air-tight. Wear can let air leak in, causing poor running. The solution is to drill the bosses, fit new bushes and renew the spindle.
[C] FLOAT CHAMBER
The H-type SU employed three sizes of chamber: T1 measuring
115/16in, T2 measuring 2 5/16in and T4 measuring 3in. This one’s is a T2 and is a semi-downdraft type, the body being mounted at 30° to the inlet manifold. The mounting banjo has the opposite 30° twist to keep the chamber vertical. The chamber lid (top of page) has two recesses shielded by a brass shroud, which allow air to escape as fuel enters.
[D] FLOAT & NEEDLE VALVE
The brass float rises on the chamber's central stud as the fuel level goes up. It pushes up the needle valve’s lever, which hinges from the lid on a pin.
The lever pushes up the needle valve, stopping fuel flow. As fuel is consumed, the float drops, the needle valve opens and more fuel enters. This is an original needle with a brass tip, which wears over time and leaks.
New types have Viton rubber tips.
[E] IDENTITY TAG This provides information on application, needle type,
spring and other useful parameters. This one is an AUC 852, as fitted to a 1957 Morris Minor 1000.
[F] LIFT PIN This is used to ensure the piston moves freely after centring the jet. Pushing it up with the engine running is also a useful tuning check: the engine revs will rise slightly then settle if the mixture’s correct, rise if it’s rich or drop if it’s lean.
[G] FILTER This brass thimble-filter seals against its seat under spring force and is the last defence against foreign bodies entering the float chamber.
This fuel connector has a barb to accept a 5/16in rubber hose.
[H] PISTON There’s a hole facing the throttle disc at the base of the piston. This transmits the low pressure between the inlet manifold and the restriction caused by the piston to the area above the piston. Atmospheric pressure enters the base of the vacuum chamber via a hole at the air filter end, below the piston. This raises the piston against its weight and spring pressure. Different types of spring are available, colour-coded by application. A venturi is formed as the piston lifts and the accelerated air creates a pressure drop across the top of the jet. Atmospheric pressure acting on the fuel in the float chamber forces it out of the jet. As the throttle is opened, airflow is increased and the pressure drop between the throttle disc and the piston becomes greater, lifting the piston further.
[I] NEEDLE The tapered needle protrudes into the jet. With the piston at its lowest position, the widest part of the needle restricts the jet’s opening, metering only a small amount of fuel. As the piston rises, a narrower part of the needle restricts the jet’s opening less, allowing more fuel to be delivered. The taper of the needle dictates the changing air-fuel ratio throughout the rev range. A wide range of needles is available, identified by different codes. This one’s a BX1.
[J] DAMPER The damper resists rapid upward movement of the piston.
This leads to a temporary increase in air-speed through the venturi when the throttle is opened quickly, which draws more fuel through the jet and prevents a flat-spot. A loose-fitting brass sleeve is fitted to the damper rod, which is immersed in SAE 20 oil. The damper is a single-acting design. The sleeve is lifted by the piston and its internal chamfer seals against its seat on the damper rod, blocking oil flow and creating resistance. Oil can pass freely though it as it drops, creating no resistance.
[K] JET ASSEMBLY The jet is screwed into the base of the body at the centre of the bridge. The jet’s static height relative to the needle sets the air-fuel ratio. The higher it is, the greater the restriction to fuelflow and the leaner the mixture.
The jet’s orifice needs to be perfectly concentric with the needle to prevent it binding, so provision is made to allow lateral adjustment. The base of the jet abuts with the fine-threaded brass mixture adjusting nut, which has a compression spring to stop it adjusting itself under vibration.
[L] CHOKE When the choke is applied, the jet lever pulls the jet downwards. This causes the jet top to correspond with a smaller diameter part of the needle, enriching the mixture. A link rod connected to the lever rotates the fast-idle cam at the same time, opening the throttle a little. The fastidle screw allows adjustment and is retained by a compression spring.