H Type SU Carburetters

What Is the H Type SU Carburetter?

The H Type is the original SU carburetter design and was produced in larger numbers than any other SU type. Introduced in the 1930s and fitted to an enormous range of British cars through to the late 1960s, the H Type established the constant-vacuum variable-choke principle that all subsequent SU designs followed.

H Types are identified by their separate float chamber, bolted to the side of the main body and connected by a fuel feed pipe. The jet is a sliding type, adjusted vertically by a jet adjusting nut beneath the carburetter body. Common sizes include the H1 (1 inch), H2 (1¼ inch), H4 (1½ inch), H6 (1¾ inch), and H8 (2 inch).

Vehicles Using H Type Carburetters

H Type SU carburetters were fitted to Austin, Morris, Wolseley, Riley, MG (pre-MGA), Singer, Standard, early Triumph models, and many others. They are also common on pre-war vehicles from manufacturers who specified SU carburetters as original equipment.

H Type Routine Servicing

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

  • Suction chamber oil: Top up with SAE 20W engine oil to within ½ inch of the top of the hollow piston rod. Essential for correct acceleration response.
  • Float chamber: Inspect the float and needle valve. Replace the needle valve if there is any sign of wear or flooding.
  • Jet centring: Check the jet is centred on the needle. A misaligned jet causes uneven needle wear and inconsistent mixture.
  • Throttle spindle: Check for play. Worn spindle bushes allow air leaks that cause erratic idling which cannot be corrected by mixture adjustment alone.

H Type Carburetter Dismantling

Before dismantling, mark the suction chamber relative to the body so it can be refitted in the same orientation. Then:

  1. Remove the air filter and disconnect the fuel feed pipe from the float chamber.
  2. Unscrew the suction chamber screws and lift the chamber clear. Take care not to damage the piston or needle.
  3. Lift out the piston. Note the needle groove position — normally flush with the underside of the piston — before removing the needle locking screw.
  4. Remove the float chamber and lift out the float and needle valve.
  5. Unscrew the jet locking nut and withdraw the jet assembly downward from the body.

H Type Reassembly

Clean all metal parts in petrol or carburetter cleaner. Never use wire to clean the jet — use a clean lint-free cloth. Replace all fibre washers and O-ring seals. Reassemble in reverse order noting:

  • Refit the jet needle with the groove flush with the bottom face of the piston.
  • Refit the suction chamber in its original orientation.
  • Jet centring is critical: slacken the jet locking nut, lower the piston onto the jet, and allow the weight of the piston to centralise the jet on the needle. Hold in position and retighten.

H Type Tuning — Single Carburetter

  1. With the engine at operating temperature, turn the jet adjusting nut clockwise to raise the jet (richen) or anticlockwise to lower it (weaken) until the engine runs at its fastest with the throttle closed.
  2. Set idle speed with the throttle stop screw (typically 500–800 rpm).
  3. Piston lift test: insert a thin rod through the piston lifting hole and raise the piston approximately 1mm. Speed should rise slightly (~25 rpm) then settle. Speed rising and staying high = too rich (lower jet). Speed immediately dropping = too weak (raise jet).

H Type Tuning — Twin Carburettors

  1. Balance the throttles first with a carburetter balance tool (Uni-Syn or similar) — both carburetters should draw equal air at idle.
  2. Adjust mixture on each carburetter using the piston lift test.
  3. Re-check balance after mixture adjustment, as changes affect idle speed.

H Type Fault Diagnosis

Engine floods or runs rich: Check float height and needle valve. A worn needle valve allows fuel to rise above the correct jet level. Check for a petrol-filled float — it will not cut off fuel supply correctly.

Engine runs lean or hesitates: Check for air leaks at the carburetter flange, jet O-ring, and throttle spindle bushes. Check the fuel pump is supplying adequate pressure.

Erratic idling: Most often caused by worn throttle spindle bushes, which can be replaced without replacing the carburetter body. Also check the piston moves freely and the suction chamber is correctly seated.

Piston sticking: Clean the chamber and piston with a lint-free cloth — do not use abrasives. A small smear of clean engine oil on the piston skirt usually resolves sticking. Check the chamber for dents or warping.

H Type Replacement Parts

All common H Type service parts are available from our store: rebuild kits, needle valves, float assemblies, jet needles, suction chamber O-rings, throttle spindle bushes, and damper assemblies for all common H Type variants.