SU Choke Units and Thermostatic Carburetters

SU Choke Units and Thermostatic Carburetters

Cold starting on petrol engines requires a richer fuel mixture than normal running, because unvaporised fuel droplets do not ignite reliably in a cold engine. SU addressed this need through several different cold-enrichment systems across their range of carburetters and ancillary equipment, including manual choke mechanisms, thermostatic choke units, and the fully automatic AED system.

Manual Choke on SU Carburetters

On most SU carburetters fitted with a conventional manual choke, the choke operates by pulling the jet downward, increasing the effective jet size and simultaneously raising the fuel level. This richens the mixture directly. A fast idle cam is linked to the choke mechanism and automatically increases idle speed when the choke is applied — necessary because a cold engine needs to run faster to avoid stalling on an enriched mixture.

The manual choke cable should move freely through its full range of travel. Common problems with manual choke operation include:

  • Stretched or frayed cable — causes incomplete choke application
  • Corroded or seized jet tube — prevents the jet from moving freely on the choke pull
  • Fast idle cam worn or adjusted incorrectly — causes high idle speed to remain after the choke is pushed in
  • Choke not returning fully — leaves the jet partially down, causing persistent rich running. Check the return spring and that the cable is not binding in its outer.

Thermostatic Choke Units

The thermostatic choke unit (also called a thermo-choke or automatic choke unit) is a self-contained unit that provides automatic cold-start enrichment without driver input. It is distinct from the AED (Automatic Enrichment Device) in that it operates the main carburetter's choke mechanism directly, rather than providing a separate enrichment circuit.

The unit consists of a bimetallic coil spring housed in a casing connected to the engine coolant system. When the engine is cold, the spring is relaxed and holds the choke mechanism in the closed (enriching) position. As coolant temperature rises, the spring contracts and progressively opens the choke. By the time the engine reaches normal operating temperature, the choke is fully open and the carburetter returns to normal operation.

The AUC 8160 Thermostatic Choke Unit

The AUC 8160 is one of the most widely known SU thermostatic choke components, used on a range of British vehicles in the 1970s. The AUC 8160 body casting is the housing that contains the bimetallic spring and links to the carburetter's choke mechanism.

New AUC 8160 body castings have been re-manufactured and are available as replacements for units where the original casting has cracked, warped, or corroded beyond repair. If you have a working bimetallic spring but a damaged body, a replacement casting allows the unit to be rebuilt without sourcing a complete assembly.

Thermostatic Choke Unit Servicing

Thermostatic choke units require relatively little maintenance but should be checked if cold-start or warm-up behaviour is poor:

  • Check coolant supply: The thermostatic unit relies on receiving hot coolant to operate correctly. A blocked coolant supply pipe means the unit never warms up and the choke remains closed — causing rich running at all temperatures. Check the pipe is connected and flowing.
  • Check the bimetallic spring: The spring can weaken over time, particularly if the engine has been overheated. A weak spring fails to fully open the choke when warm. Inspect and replace if necessary.
  • Check the linkage: The linkage between the thermostatic unit and the carburetter choke mechanism must move freely. Corrosion or seized pivots cause erratic or incomplete choke operation.
  • Check the housing for leaks: If the coolant passages within the housing are leaking, coolant will escape and the unit will not warm properly. Check all connection points.

Thermostatic Choke Unit Fault Diagnosis

Engine runs rich when warm / will not come off choke: The most common fault. The coolant supply to the thermostatic unit is blocked, the bimetallic spring is weak or broken, or the choke linkage is seized in the closed position. Confirm by checking coolant supply first — it's the most common cause and the easiest to diagnose.

Poor cold starting / engine stalls when cold: The choke is not closing fully when cold. Check the adjustment of the thermostatic unit — there is usually a calibration mark that indicates the correct position of the housing relative to the adjustment collar. The housing can be rotated slightly to change the cold-setting position of the choke.

Choke permanently stuck partially open: The bimetallic spring has taken a set (permanently deformed). This typically happens after severe overheating. The spring must be replaced — the spring alone is available as a service part on most applications.

Coolant leak from thermostatic unit: Replace the sealing rings around the coolant connections. If the body casting itself is cracked, replacement with a new AUC 8160 body or equivalent is required.

Adjustment and Calibration

Thermostatic choke units have a calibration setting that determines the position of the choke when the engine is cold. The housing can usually be rotated slightly within its mounting collar and locked in position with a screw. The correct cold position places the choke fully closed when the engine is at ambient temperature. If the vehicle is used in very cold climates, the calibration may need to be advanced slightly; in warm climates, retarded slightly.

After any adjustment, confirm correct operation by starting the engine from cold and observing that the choke progressively opens over 5–10 minutes of normal idle. The engine should be running cleanly on the normal mixture within 10 minutes of starting from a cold soak at typical UK ambient temperatures.

Conversion to Manual Choke

Where thermostatic choke components are unavailable or uneconomical to repair, conversion to a manual choke cable and mechanism is a practical alternative. The conversion typically involves fitting a manual choke jet (for HS and HIF types), a cable bracket, and a suitable Bowden cable. The fast idle cam and linkage remain in use. We can advise on the correct conversion parts for your application.

Choke Unit and Thermostatic Parts Available

We stock replacement parts for SU choke and thermostatic systems including bimetallic springs, AUC 8160 body castings, coolant connection seals, choke cable assemblies, fast idle cams, and complete thermostatic choke unit assemblies where available. Contact us if you need assistance identifying the correct parts for your vehicle.