HS Type SU Carburetters

What Is the HS Type SU Carburetter?

The HS Type is the most widely encountered SU carburetter on classic British cars of the 1960s and 1970s. Fitted as standard equipment to MG, Triumph, Austin-Healey, Austin, Morris, and many other marques, the HS Type was the production mainstay of British motor industry carburation for over two decades.

The HS (Horizontal type, Slimline body) shares the same constant-vacuum variable-choke principle as all SU designs but incorporates several improvements over the older H and HD Types, including a more compact body, a revised jet adjustment system, and an improved float chamber design. The jet is adjusted by a hexagonal nut directly accessible from below the body — simpler and more reliable than the HD Type's diaphragm arrangement.

Common HS Type Applications

HS Types are found on an extremely wide range of vehicles. Common applications include:

  • MG MGB, MGC, Midget — twin HS4 (1½ inch) on the MGB, twin HS6 (1¾ inch) on the MGC and twin HS2 (1¼ inch) on the Midget
  • Triumph TR3, TR4 and Spitfire — twin HS6 on the TR3 and TR4, twin HS2 on the Spitfire
  • Austin-Healey 3000 and Sprite — HS4, HS6, HD6 or HD8 on the 3000 depending on model, and twin HS2 on the Sprite
  • Rover P6 2000 — single HS6, or twin HS8 on the 2000 TC
  • Austin/Morris 1100, 1300 and 1800 — single HS2 on the 1100 and 1300, single HS6 on the 1800
  • Mini Cooper and Cooper S — twin HS2 (1¼ inch)
  • Jaguar XJ6 (early models) — twin HS8 (2 inch) carburettors

HS Type Identification

HS Types are designated by bore size: HS2 (1¼ inch), HS4 (1½ inch), HS6 (1¾ inch), and HS8 (2 inch). The designation is usually stamped on the body or on a tag. Look for the slim, neat body with the float chamber mounted either to the rear or side, and the jet adjusting nut accessible from directly below the body.

HS Type Routine Servicing

Service every 6,000 miles or annually. The procedure is very similar to the H Type:

  • Suction chamber oil: Remove the damper cap and top up with SU damper oil (SAE 20) until the level is just below the top of the hollow piston rod. See our damper oil guide.
  • Jet needle: Check for wear by inspecting the needle where it contacts the jet. A worn needle shows a bright line or groove on one side. A worn needle causes inconsistent mixture and should be replaced.
  • Float chamber and needle valve: Check the needle valve for wear and replace if the engine has been flooding. Inspect the float by shaking it gently — any fuel inside indicates a leak and the float must be replaced.
  • Throttle spindle: Check for free play. Worn spindle bushes are a common cause of erratic idling on high-mileage HS carburetters. The correct repair is to ream the body in line and fit modern Teflon-coated bushes — see our spindle bush guide.
  • Jet O-ring: The jet is sealed by a small rubber O-ring. This can harden and crack with age, causing fuel leaks and lean running. Replace as part of a routine service.

HS Type Carburetter Dismantling

  1. Mark the suction chamber position relative to the body before removal.
  2. Remove the damper cap and withdraw the damper assembly.
  3. Unscrew the suction chamber retaining screws and lift off the chamber.
  4. Remove the piston. Note the needle groove position — standard position is flush with the underside of the piston, but some vehicles use a raised or lowered needle for tuning purposes. Record the position before removing the needle locking screw.
  5. Remove the float chamber. On most HS Types this is held by a single bolt through the base.
  6. To remove the jet, disconnect the jet linkage and unscrew the jet tube sleeve nut from the float chamber, then withdraw the jet downward. Unscrew the jet locking nut to remove the jet bearing.

HS Type Reassembly

Replace all O-rings, the jet bearing O-ring, and all fibre washers. Key points:

  • Refit the needle with the groove in the same position as noted during dismantling.
  • Refit the suction chamber in its original orientation — the scribe marks made before dismantling ensure correct alignment.
  • Jet centring: With the jet and bearing fitted but the locking nut slightly loose, lower the piston. The needle should self-centralise in the jet under piston weight. Hold and retighten.
  • Check float height after refitting the float chamber. The correct setting depends on the float and needle valve fitted (for example a 5/16in test bar for a brass float, or a 0.125–0.187in gap for the later nylon float) — see our float level settings guide.

HS Type Tuning — Single Carburetter

  1. With the engine fully warm, ensure idle speed is approximately 800 rpm.
  2. Turn the jet adjusting nut: up raises the jet and weakens the mixture; down lowers the jet and richens it. Adjust until the engine runs at its fastest at idle.
  3. Reset idle speed if necessary.
  4. Piston lift test: lift the piston approximately 1mm. The engine speed should rise slightly and then settle. Speed rising and staying high = mixture too rich (raise the jet by turning the nut up). Speed immediately dropping = mixture too weak (lower the jet by turning the nut down).

HS Type Tuning — Twin Carburettors

  1. Balance the throttles first using a Uni-Syn or similar air flow meter. Both carburetters must draw equal air at idle before mixture is adjusted.
  2. Connect the throttle linkage to ensure both throttles open simultaneously and by the same amount.
  3. Adjust mixture on each carburetter independently using the piston lift test.
  4. Re-check synchronisation after adjusting mixture.

HS Type Fault Diagnosis

Rich running / high fuel consumption: Check the needle valve and float height. A stuck-open needle valve or punctured float causes the fuel level to rise above the jet, flooding the mixture. Also check the needle position — if it has moved up in the piston (richer position) it will cause persistent over-fuelling.

Lean running / flat spots: Look for air leaks at the carburetter flange gasket, jet O-ring, and throttle spindle bushes. Even a small air leak bypasses the carburetter's metering and causes very lean running at idle and light load.

Difficult cold starting: Check the choke linkage pulls the jet fully down when the choke is operated. Check the fast idle cam engages correctly to raise idle speed when the choke is applied.

Oil consumption from dashpot: If the damper oil needs frequent topping up, inspect the damper O-ring and the suction chamber for cracks. A cracked chamber will also allow oil to enter the airstream.

HS Type Replacement Parts

We stock a comprehensive range of HS Type parts including full rebuild kits for HS2, HS4, HS6, and HS8 carburetters, needle valves, floats, jet needles, jet bearing O-rings, throttle spindle bush kits, and suction chamber repair kits. Service kits for both single and pair applications are available.