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 — HS4 (1½ inch) and HS6 (1¾ inch) in single and twin configurations
  • Triumph TR4, TR5, TR6, Spitfire, GT6 — HS6 twin carburettors on most six-cylinder models
  • Austin-Healey 100, 3000, Sprite — HS4 or HS2 depending on model and year
  • Rover 2000, P6 — HS6 twin carburettors
  • Austin/Morris 1100, 1300, 1800 — HS4 single carburetter
  • Mini Cooper and Cooper S — HS2 (1 inch) single or twin
  • Jaguar XJ6 (early models) — HS8 (2 inch) triple carburettors

HS Type Identification

HS Types are designated by bore size: HS2 (1 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 SAE 20W engine oil to within ½ inch of the top of the damper rod. Some owners prefer ATF (automatic transmission fluid) for a slightly different damping characteristic — either is acceptable.
  • 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. Replacement bush kits are available and can be fitted with simple hand tools.
  • 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, unscrew the jet locking nut from below and withdraw the jet upward through the body. The jet bearing and O-ring will come away with it.

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. Correct float height varies by application — check the workshop manual or set to 19mm (¾ inch) below the float chamber mating face as a general 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: clockwise raises the jet and richens the mixture; anticlockwise lowers the jet and weakens 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 (lower the jet by turning the nut anticlockwise). Speed immediately dropping = mixture too weak (raise the jet by turning clockwise).

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 mechanism operates fully, closing the jet down completely. 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.