Introduction to SU Carburetters

Basic Principles of SU Carburetters

SU carburetters (the spelling used by the original manufacturer) are among the most elegant pieces of engineering on any classic British car. Simple in concept, precise in operation, and remarkably durable when properly maintained — they remain the carburetter of choice for millions of classic vehicle owners and restorers worldwide.

How the Variable Choke Design Works

All carburetters work by using a venturi — a narrowing in an air passage — to create a pressure drop that draws fuel from a float chamber into the airstream. What makes the SU design distinctive is its variable choke principle.

Most carburetters use a fixed choke (a fixed-size venturi), which means the mixture strength varies with engine speed and requires additional circuits to compensate. The SU uses a sliding piston — called the suction piston — that rises and falls automatically in response to engine demand. As the piston rises, the choke area increases. As it falls, it decreases. The result is a constant depression (vacuum) across the jet, regardless of engine speed or load. This is why SU carburetters are also known as constant vacuum carburetters.

Because the depression at the jet is always the same, fuel delivery is governed purely by jet needle position. This gives precise, progressive mixture control with very few moving parts — no accelerator pump, no power valve, no separate idle circuit.

The Suction Piston and Dashpot

Inside the suction chamber above the piston, engine vacuum acts on the upper face of the piston. Below it, atmospheric pressure acts via the jet and needle assembly. The piston finds its own equilibrium point for any given engine condition — rising under light load and high speed, dropping under heavy load and low speed.

A tapered needle is attached to the centre of the piston. As the piston rises, the needle rises with it, progressively withdrawing from the jet and allowing more fuel to flow. The needle profile is precisely calibrated — different needle codes give different mixture characteristics across the rev range, allowing fine tuning for different engine specifications.

A small dashpot above the piston is filled with light oil. This dampens the piston's upward movement, providing a momentary richening effect during acceleration — acting like an accelerator pump, but without a separate component.

The Float Chamber

Fuel level in the jet is controlled by a float chamber. A hinged float rises with the fuel level and closes a needle valve when the correct level is reached. Maintaining correct float height is critical: too high and the mixture runs rich and fuel may flood the engine; too low and the engine runs lean and may hesitate under load.

Types of SU Carburetter

SU produced several families of carburetter over their long history. The main types found on classic vehicles are:

  • H Type — the original and most widely produced design, fitted to a vast range of British cars from the 1930s through to the 1960s. Identified by its separate float chamber mounted to the side of the main body.
  • HD Type — an evolution of the H Type featuring a diaphragm-operated jet rather than the sliding jet of the H Type. Often found paired with an Auxiliary Enrichment Carburetter (AEC) for cold starting.
  • HS Type — the most common type on classic British cars of the 1960s and 1970s, including MG, Triumph, Austin-Healey, and many others. Similar in layout to the H Type but with improvements including a revised jet adjustment system.
  • HIF Type — the Horizontal Integral Float chamber design, developed to meet exhaust emission regulations. The float chamber is built into the main body casting rather than mounted separately, making the unit more compact.

Why SU Carburetters Are Still Popular

Decades after production ended, SU carburetters remain the preferred choice for classic car owners. They are simple to work on — no complex emulsion tubes, no power valves, and no multiple jets to clean. Tuning is straightforward: mixture is adjusted by a single jet screw, and needle selection allows precise calibration. Parts availability is excellent, with a full range of service kits, needles, float valves, and jet assemblies still manufactured and readily available.

When correctly set up, an SU carburetter delivers fuel economy and performance that rivals modern fuel injection systems for its era. The key is correct maintenance — clean jets, correct float height, a good needle valve, and the right needle for your engine specification.

Key Maintenance Points

  • Replace the needle valve if the float chamber floods or fuel level is inconsistent
  • Check and replace the jet diaphragm or O-ring to prevent mixture leaks
  • Keep the suction chamber oil topped up for smooth dashpot damping
  • Check float height and adjust as necessary
  • Inspect throttle spindle bushes for wear — worn bushes cause air leaks and erratic idling
  • Select the correct jet needle for your engine specification

The full range of SU carburetter parts and rebuild kits are available from our online store.