MGB Carburettor Rebuild

MGB Carburettor Rebuild — Parts & Guide

The MGB is one of the most popular British classics in the UK, and its SU carburettors are well-understood, rebuildable, and fully supported by a complete genuine parts range. Whether you're restoring a chrome bumper twin-HS4 car or a rubber bumper single-HIF44 model, SU Specialists has everything you need for a correct, lasting MGB carburettor rebuild.

Which Carburettor Does My MGB Have?

Carburettor fitment changed across the MGB's long production run (1962–1980):

  • 1962–1974 (chrome bumper): Twin SU HS4 carburettors (1½ inch). These cars run twin carbs in parallel — both should be rebuilt and balanced together.
  • 1974–1980 (rubber bumper): Single SU HIF44 carburettor. The HIF44 has an integral float chamber and temperature-compensating jet, requiring different parts to the HS4.
  • Some US-spec and later cars may have had Zenith-Stromberg or other carburettors fitted as standard or as replacements — check before ordering.

MGB HS4 Rebuild Parts

For chrome bumper MGBs with twin HS4 carburettors, you'll need parts for both units. Key items:

  • HS4 Major Repair Kit — gaskets, O-rings, float valve and seals. Order two for a twin-carb rebuild.
  • HS4 Needles — standard MGB fitment is typically the AAA needle, but check the needle code on your existing units. Both carbs should run the same needle.
  • HS4 Jet Assemblies — replace in pairs for consistent mixture across both cylinders.
  • Pistons & Suction Chambers — inspect for scoring or corrosion. Replace as matched pairs if worn.

MGB HIF44 Rebuild Parts

For rubber bumper MGBs with the single HIF44:

  • HIF44 Major Repair Kit — all seals and consumables for the HIF44 body and integral float chamber.
  • HIF44 Needles — standard fitment is typically BAF or BBT. Verify the needle code on your unit.
  • HIF44 Jet Assembly — temperature-compensating assembly; replace if the bi-metal has failed or the jet is worn.

MGB Carburettor Rebuild — What to Expect

A thorough MGB carburettor rebuild should address all of the following:

  1. Full strip and clean — remove all old fuel deposits and varnish from drillings and chambers
  2. Inspect throttle spindle — check for side-play; worn spindles must be replaced to eliminate air leaks
  3. Replace all gaskets and O-rings — do not reuse old rubber seals regardless of apparent condition
  4. Replace jet and needle together — a new needle in a worn jet (or vice versa) will not restore correct mixture
  5. Set float height — critical for fuel level; follow the correct height for your specific unit
  6. On twin-carb cars: synchronise and balance — both carbs must have identical airflow and mixture settings
  7. Set idle mixture and speed — use a CO meter or colortune for best results

Full step-by-step instructions are in our SU carburettor rebuild guide. For tuning guidance after rebuild, see our SU tuning guide.

SU Carburettor Refurbishment Service

Prefer to have the work done for you? Our SU Carburettor Refurbishment Service returns your unit to as-new condition, fully tested and ready to refit. We cover both HS4 and HIF44 units.

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