HD Type SU Carburetters

What Is the HD Type SU Carburetter?

The HD Type SU carburetter is an evolution of the original H Type, featuring a diaphragm-operated jet rather than the sliding jet of its predecessor. Produced from the 1950s onwards, HD Types were fitted to a range of British cars including Austin, Morris, Wolseley, and others in the BMC and BL family.

The main visual distinction between the HD and H Type is the jet assembly: where the H Type uses a sliding brass jet adjusted by a nut beneath the body, the HD Type uses a rubber diaphragm to move the jet up and down in response to the mixture adjustment screw. This design is more resistant to fuel leakage and easier to seal, but the diaphragm itself requires periodic replacement.

HD Types are also frequently found paired with an Auxiliary Enrichment Carburetter (AEC) for cold starting, which replaces the traditional manual choke. The AEC is a small separate unit mounted to the inlet manifold that provides a rich mixture during cold running and automatically leans out as the engine warms up.

HD Type Identification

HD Types are identified by the letter-number designation stamped on the carburetter body: HD4 (1½ inch), HD6 (1¾ inch), and HD8 (2 inch) are the most common. The body shape is similar to the H Type but look for the diaphragm housing below the jet area — this is the key distinguishing feature.

HD Type Routine Servicing

Service every 6,000 miles or annually. Key maintenance items:

  • Suction chamber oil: Top up with SAE 20W engine oil to within ½ inch of the top of the damper rod. This is the same procedure as the H Type.
  • Jet diaphragm: The diaphragm is the most common failure point on HD Types. Inspect for cracks, tears, or hardening. A failed diaphragm causes rich running, fuel leakage, and poor idle. Replace as part of the standard service kit.
  • Float chamber and needle valve: Inspect and replace the needle valve if flooding is occurring. Check float integrity.
  • Throttle spindle: Check for play. Worn spindle bushes allow uncontrolled air entry and cause erratic idling.

HD Type Carburetter Dismantling

  1. Mark the suction chamber in relation to the body before removal.
  2. Unscrew the suction chamber screws and lift the chamber. Lift out the piston and note the needle position before removing it.
  3. Remove the float chamber.
  4. To access the jet diaphragm, unscrew the jet housing from the underside of the carburetter body. The diaphragm will be visible. Note how the jet is positioned relative to the diaphragm before dismantling further.
  5. Withdraw the jet assembly and diaphragm for inspection and replacement.

HD Type Reassembly

Replace all rubber diaphragms, O-rings, and fibre washers as a matter of course. When refitting the jet diaphragm, ensure it seats correctly in its groove without distortion — a poorly seated diaphragm will leak immediately.

As with the H Type, centring the jet on the needle is critical. With the jet assembly fitted but the locking screw slightly loose, lower the piston onto the jet and allow it to self-centralise under its own weight before tightening.

HD Type Tuning

HD Type tuning follows the same procedure as the H Type:

  1. With the engine at operating temperature, adjust the mixture screw until the engine runs fastest at idle.
  2. Set idle speed with the throttle stop screw.
  3. Use the piston lift test to confirm mixture: lift the piston approximately 1mm. A slight speed increase then settling indicates correct mixture. Rising and staying high = too rich. Immediately dropping = too weak.

For twin HD Type installations, synchronise the throttles before adjusting mixture on each carburetter.

The Auxiliary Enrichment Carburetter (AEC)

When fitted, the AEC replaces the conventional manual choke. It consists of a small bi-metallic thermostat connected to the inlet manifold water temperature. When the engine is cold, the AEC opens a bypass circuit that provides additional fuel. As the engine warms, the thermostat progressively closes the circuit.

AEC servicing involves checking the bypass circuit for blockage, verifying the thermostat response, and ensuring the unit is airtight. A stuck-open AEC will cause persistent rich running and high fuel consumption. A stuck-closed AEC results in difficult cold starting.

HD Type Fault Diagnosis

Fuel leaking from below the carburetter: Almost always a failed jet diaphragm. Replace the diaphragm as part of a full service kit.

Rich running / black smoke: Check the diaphragm, float height, and needle valve. If an AEC is fitted, check it is closing fully when the engine is warm.

Erratic cold starting: If an AEC is fitted, check the thermostat operation and AEC bypass circuit. If no AEC is fitted, check the choke mechanism is operating correctly.

Flat spot on acceleration: Check the suction chamber oil level — this is the HD Type's equivalent of an accelerator pump and must be correctly filled for smooth acceleration.

HD Type Replacement Parts

Our store stocks full service and rebuild kits for HD Type carburetters, including replacement diaphragms, needle valves, float assemblies, jet needles, and throttle spindle bushes. AEC service parts are also available where applicable.