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 surrounds the jet tube and is pivoted along a line parallel to the inlet flange.
Common HIF Type Applications
HIF carburetters were fitted to later British Leyland and Rover Group vehicles, typically from the mid-1970s onwards:
- MGB (from 1975) — HIF4 (1½ inch) on US-specification and later UK models
- Triumph TR7, TR8 — HIF6 twin carburetters
- Rover SD1 — HIF6 carburetters
- Austin/Morris Allegro, Maxi — HIF4 and HIF6 variants
- Mini (later models) — HIF38 and HIF44 variants
- Land Rover Series III and early Defender — HIF6 on 2.25 petrol engines
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 used HIF4, HIF5, HIF6 in the older notation.
HIF Type Routine Servicing
Key service items for the HIF Type:
- Suction chamber oil: Top up as with other SU types — SAE 20W engine oil to within ½ inch of the damper rod top.
- 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. The float in HIF Types surrounds the jet tube, so inspect it carefully for damage or fuel contamination.
- Jet assembly: The jet in the HIF Type is adjusted by a thumbwheel on the side of the body, which rotates a cam to move the jet. Check the thumbwheel turns smoothly. The jet O-ring seals the jet tube — replace if any sign of weeping fuel.
- Mixture adjustment screw: Unlike earlier types, the HIF uses a tamper-proof mixture adjustment screw covered by a pressed steel cap. This can be removed for adjustment during servicing.
- Temperature compensator: Many HIF carburetters include a bimetallic temperature compensator mounted in the air intake side of the body. This provides an additional air bleed when the engine is hot to compensate for fuel vaporisation. Check it moves freely and is not stuck.
HIF Type Carburetter Dismantling
- Mark the suction chamber position before removal.
- Remove the damper cap and suction chamber screws. Lift the chamber and withdraw the piston and needle assembly.
- 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.
- The jet is accessed from the side. Remove the jet adjustment assembly to withdraw the jet for inspection and cleaning.
- Do not dismantle the temperature compensator unless replacement is specifically required — it is factory-calibrated and not normally adjusted in service.
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 that the float sits correctly around the jet tube after assembly. The float must move 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 but uses the side-mounted thumbwheel and the mixture adjustment screw:
- With the engine at operating temperature and idling, adjust the throttle stop screw to give the correct idle speed.
- Adjust the mixture via the mixture screw (remove the tamper-proof cap if necessary). Turning clockwise weakens, anticlockwise richens — opposite to many other carburetter types.
- 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.
- 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 float height via the bottom cover plate — with the cover removed, the fuel level should be at the bottom of the float chamber. Check the needle valve. Check the temperature compensator is not stuck closed (which would block the air bleed that leans the mixture when hot).
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: Hot running rich or hesitation when warm is often the temperature compensator stuck or non-functional. 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, HIF5, and HIF6. Kits include replacement gaskets, O-rings, needle valves, float assemblies, jet needles, and temperature compensator components where available.