HIF Type SU Carburetters

What Is the HIF Type SU Carburetter?

The HIF (Horizontal Integral Float chamber) carburetter is SU's most sophisticated design, developed primarily to meet increasingly strict exhaust emission regulations from the early 1970s onwards. Functionally it uses the same constant-vacuum variable-choke principle as all SU types, but with several important design differences that make it more compact, better sealed, and better suited to emission-controlled applications.

The most significant difference is that the float chamber is built into the main body casting rather than mounted as a separate unit. This eliminates the external fuel pipe connection and reduces the risk of fuel evaporation, making the HIF better suited to meeting vapour emission standards. The float sits in the chamber beneath the body and pivots on a spindle, with access through a bottom cover plate.

Common HIF Type Applications

HIF carburetters were fitted to later British Leyland and Rover Group vehicles, typically from the mid-1970s onwards. Examples include:

  • MGB (later UK models) — twin HIF4 (1½ inch)
  • Jaguar XJ6 4.2 (Series 2) — twin HIF7
  • Range Rover V8 (1980s) — twin HIF44
  • Mini and Metro (later models) — single HIF38 or HIF44 depending on engine

Identifying HIF Type Carburetters

HIF Types are distinguished by their compact, self-contained appearance — no external float chamber. The bottom cover plate gives access to the integral float chamber. HIF designations use the bore size in millimetres rather than fractional inches: HIF38 (38mm, approximately 1½ inch) and HIF44 (44mm, approximately 1¾ inch) are the most common. Earlier designations such as HIF4, HIF6 and HIF7 used the older notation.

HIF Type Routine Servicing

Key service items for the HIF Type:

  • Suction chamber oil: Top up with SU damper oil (SAE 20) to just below the top of the hollow piston rod (within 6.5mm/¼in on later ball-bearing chambers). See our damper oil guide.
  • Bottom cover plate and float: Remove the bottom cover plate to access the float chamber. Inspect the float, the float pivot spindle, and the needle valve. Inspect the float carefully for damage or fuel contamination.
  • Jet assembly: The HIF jet is moved by the jet adjusting screw on the side of the body, acting through a lever with a bi-metal strip that compensates for fuel temperature. Check the jet head moves freely in the bi-metal cut-out. The jet O-ring seals the jet tube — replace it if there is any sign of weeping fuel.
  • Jet adjusting screw seal: On cars built to emission regulations the jet adjusting screw may be covered by a tamper-proof seal or cap, which has to be removed for adjustment.
  • Float level: With the body inverted so the needle valve is held shut by the float, the float should sit 1.0mm ± 0.5mm below the float chamber face. See our float level settings guide.

HIF Type Carburetter Dismantling

  1. Mark the suction chamber position before removal.
  2. Remove the damper cap and suction chamber screws. Lift the chamber and withdraw the piston and needle assembly.
  3. Remove the bottom cover plate. The float, pivot spindle, and needle valve are now accessible. Remove the spindle to release the float and needle valve.
  4. The jet is accessed from the side. Remove the jet adjustment assembly to withdraw the jet for inspection and cleaning.
  5. Note the position of the bi-metal jet lever before removing it, so it goes back in the same way.

HIF Type Reassembly

Replace the bottom cover plate gasket, jet O-ring, and all other sealing components. Key points:

  • The float pivot spindle screws into the body casting — do not overtighten. A drop of thread sealant helps prevent fuel seepage around the spindle.
  • After refitting the jet, centre it as with other SU types: slightly loose, lower the piston, allow self-centring, retighten.
  • Check the float level and that the float pivots freely through its full range without binding.
  • Refit the bottom cover plate with a new gasket. Tighten evenly to avoid distortion.

HIF Type Tuning

HIF tuning is similar to other SU types, using the jet adjusting screw on the side of the body:

  1. With the engine at operating temperature and idling, adjust the throttle stop screw to give the correct idle speed.
  2. Adjust the mixture with the jet adjusting screw (remove any tamper-proof seal first). Turning it down/clockwise richens; up/anticlockwise weakens. Turn it slowly until the fastest idle speed is reached.
  3. Use the piston lift test to confirm mixture: lift the piston 1mm. Speed should rise slightly and settle. Staying high = too rich. Immediately dropping = too weak.
  4. For twin HIF installations, balance airflow first, then adjust each carburetter's mixture independently.

HIF Type Fault Diagnosis

Fuel leaking from bottom of carburetter: The bottom cover plate gasket or the float chamber O-ring is leaking. Remove and replace the cover plate gasket. Check the cover plate is not warped.

Rich running: Check the float level: with the body inverted, the float should sit 1.0mm ± 0.5mm below the float chamber face. Check the needle valve. Check the jet head moves freely in the bi-metal lever cut-out.

Lean running or air leaks: Check all O-ring seals, particularly the jet O-ring and the carburetter mounting gasket. The HIF's more complex internal sealing means there are more potential leak points than simpler designs.

Temperature-related problems: Rich running or hesitation when hot can be the bi-metal jet lever binding, so the jet is not compensating for fuel temperature. Cold starts with rich running that doesn't clear suggests the cold-start enrichment circuit is stuck open.

HIF Type Replacement Parts

We stock service and rebuild kits for all common HIF variants including HIF38, HIF44, HIF4, HIF6 and HIF7. Kits include replacement gaskets, O-rings, needle valves, float assemblies and jet needles.