HS4 Rebuild Service
HS4 Carburettor Rebuild Service & Parts
The HS4 is one of the most widely fitted SU carburettors on British classic cars — found on the MGB, Triumph Spitfire, Morris Minor 1000 and many others. SU Specialists carries the complete range of genuine OEM-specification parts for a full HS4 rebuild
About the SU HS4
The HS4 is a 1.5-inch (38mm) fixed-needle, variable-jet carburettor. Its constant-depression design means mixture quality remains consistent across all engine speeds — but only when all components are within tolerance. Common problems on worn HS4 units include rich or weak running caused by a worn jet, flat spots from a faulty damper, and air leaks at a worn throttle spindle.
HS4 Rebuild Parts Available
- HS4 Major Repair Kit — the essential starting point. Contains all gaskets, O-rings, float valve, washers and seals for a complete rebuild.
- HS4 Needles — standard and alternative profiles including AAA, AAB, ABD, ADE and more. If you're retuning or correcting a rich/weak condition, the needle profile is the first place to look.
- HS4 Jet Assembly — genuine OEM jet to restore correct fuel metering. Replace alongside the needle for best results.
- HS4 Piston & Suction Chamber — matched assemblies with the correct piston weight and suction chamber bore. A damaged or corroded suction chamber is a common cause of mixture problems that no amount of adjustment can fix.
- Float Chamber & Gasket — HS4 float chamber with correct float and needle valve. Ensures accurate fuel level control.
- Throttle Spindle Kit — eliminates air leaks at the spindle that lean the mixture unpredictably.
- Damper & Oil — hydraulic damper assembly and correct-viscosity damper oil for smooth, flat-spot-free acceleration.
HS4 Rebuild Step-by-Step
Before ordering, identify your HS4 by the casting number on the carburettor body and the needle code stamped on the needle shank. This ensures you order the correct needle profile and jet size. Our identification guide walks you through this in detail.
For the rebuild procedure itself, see our SU carburettor rebuild guide — it covers stripping, cleaning, reassembly and initial setup for all HS-series carburettors including the HS4.
HS4 Applications
The HS4 was fitted to a wide range of British classics, including:
- MGB (1962–1974, twin HS4 setup)
- Triumph Spitfire Mk3 and Mk IV
- Morris Minor 1000 (later models)
- Austin-Healey Sprite Mk III and Mk IV
- MG Midget (early models)
- Mini Cooper S (some variants)
Always verify against your chassis number and factory build record — carburettor fitment changed throughout production runs.