SU AED — Automatic Enrichment Device

What Is an SU AED?

The SU AED (Automatic Enrichment Device) is a thermostatic cold-start enrichment system developed by SU to replace the manual choke on certain applications. Rather than requiring the driver to manually operate a choke lever, the AED uses engine coolant temperature (via a bimetallic thermostat) to automatically provide a rich mixture during cold starting and progressively lean it out as the engine warms up.

AEDs were fitted to a range of British cars in the 1960s and 1970s, particularly where packaging constraints made a manual choke inconvenient or where emissions regulations encouraged automatic enrichment systems. They are found on some HD Type installations and on certain HS Type applications, typically mounted to the inlet manifold alongside the main carburetter(s).

How the AED Works

The AED consists of a small housing containing a bimetallic spring thermostat that is in contact with engine coolant via the cooling system. A small valve or jet within the AED unit controls a bypass circuit that delivers additional fuel to the inlet manifold when open.

When the engine is cold, the bimetallic spring holds the enrichment valve open, allowing extra fuel through the bypass circuit. As the engine coolant temperature rises, the spring gradually contracts and closes the valve, cutting off the additional fuel supply. The entire process is automatic — there is no driver input required once the engine has been started.

On installations with an AED, the main carburetter(s) may be jetted slightly leaner than standard, as the AED provides the additional fuel needed for cold operation. This means a carburetter from an AED-equipped vehicle should not be swapped directly onto a non-AED installation without re-jetting.

Identifying an AED Installation

The AED is a small separate unit, typically about the size of a large relay, mounted on or near the inlet manifold. It has connections for:

  • A coolant supply pipe (usually taken from the heater circuit or cylinder head)
  • A fuel supply pipe (from the carburetter float chamber or fuel supply line)
  • A manifold connection (a short stub pipe or bore into the manifold)

On some applications a separate small jet unit, mounted in the inlet manifold, provides the fuel delivery point for the AED. If the AED and its manifold connection are removed without blanking off the manifold bore, a significant air leak results.

AED Routine Servicing

AED units require relatively little routine maintenance, but they should be inspected whenever poor cold-start behaviour is reported or when persistent rich running occurs after the engine is warm.

  • Check the thermostat response: With the engine cold, the AED valve should be fully open. As the engine warms, it should progressively close and be fully closed within 5–10 minutes of normal operation.
  • Inspect fuel connections: Check the fuel supply pipe and fittings for leaks or deterioration. A seeping connection will cause persistent rich running.
  • Check the coolant connection: Ensure the coolant pipe is connected and not blocked. A blocked coolant supply means the thermostat never receives heat signal, and the AED remains open permanently — causing continuous rich running.
  • Inspect the jet or orifice: The small fuel orifice within the AED or its manifold fitting can become blocked by fuel varnish. Clean with carburetter cleaner.

AED Dismantling

  1. Disconnect the coolant pipe — have a container ready to catch any coolant spill. Re-seal the coolant circuit immediately.
  2. Disconnect the fuel supply pipe.
  3. Unscrew the AED from its mounting. Note the orientation relative to the manifold fitting.
  4. Remove the cover of the AED unit to access the thermostat and valve assembly. On most designs, the cover is secured by two or three small screws.
  5. Carefully remove the bimetallic spring and valve assembly. These are precision components — handle with care and note exactly how they are assembled before separation.

AED Inspection

With the unit dismantled, check:

  • The bimetallic spring for distortion, damage, or loss of tension. A spring that has been overheated (for example, by accidentally running without coolant) may be permanently deformed and will not close properly.
  • The valve seat and needle for wear or debris. Any roughness on the needle or seat will prevent proper closure when warm.
  • All sealing surfaces for flatness and freedom from corrosion.
  • The fuel jet or orifice — check it is clear by blowing through with compressed air.

AED Reassembly

  1. Clean all components and replace any seals or gaskets in the service kit.
  2. Refit the bimetallic spring and valve assembly in the correct orientation — incorrect fitting will reverse the operation of the unit (open when hot, closed when cold).
  3. Refit the cover and tighten evenly.
  4. Refit to the manifold. Reconnect the fuel and coolant pipes and check all connections for tightness.

AED Fault Diagnosis

Rich running when engine is warm / black smoke / high fuel consumption: The AED valve is not closing fully. Check the coolant supply pipe is connected and passing hot coolant. If coolant supply is confirmed, the thermostat spring may be damaged or the valve is fouled with debris — dismantle and inspect.

Poor cold starting / engine won't run cold without manual choke: The AED valve is stuck closed or the fuel supply to the AED is blocked. Check the fuel supply pipe and the jet or orifice for blockage.

Intermittent rich/lean behaviour: A partially blocked coolant pipe can cause the thermostat to respond erratically. Check for flow through the coolant supply pipe. Also check that the fuel supply pipe is not kinked or partially blocked.

Coolant leak from AED: The seals around the coolant connection have failed. These can be replaced with the AED in situ on some designs, or the unit may need to be removed for a full reseal.

AED Tuning

Some AED units have a limited adjustment range for the thermostat calibration point. This can be used to fine-tune the warm-up richening period — advancing (opening) the calibration point means the AED closes sooner; retarding it means the unit enriches for longer. Consult the specific workshop manual for your vehicle before attempting this adjustment, as it varies by application.

AED Replacement Parts and Alternatives

Service kits for AED units include replacement seals, valve needles, and gaskets where available. On some vehicles where AED components are no longer available, owners convert to a manual choke conversion — a relatively straightforward modification that replaces the AED with a conventional choke jet and cable operation. We can advise on the correct conversion kit for your application.