The SU carburettor is a reliable and elegant design, but its internal components are service items that wear over time and must be matched to the correct carburettor type. The MGB used two principal SU carburettor types across production, the HS4 and the HIF4, and the internal components differ between them. Identifying which type is fitted is the first step when ordering any replacement parts.
HS4 Components
The HS4 carburettor was standard fitment from 1962, continuing on home market cars through to late 1973 when the HIF4 replaced it. The HS4 features an external float chamber connected to the carburettor body by a fuel feed pipe.
Early HS4 carburettors used fixed needles, while later examples fitted from 1971 on home market 18V-series engines adopted spring-loaded biased needles. This distinction affects not only the needle itself but also the needle locking screw, jet bearing kit, and needle guide, all of which differ between fixed and biased needle installations. The HS4 throttle disc was originally a plain type, though later HS4 specifications used an overrun valve disc, a plain disc replacement kit is available for more efficient airflow.
HIF4 Components
The HIF4 carburettor replaced the HS4 on export cars from 1972 and on home market cars from late 1973. It shares the same 1½-inch choke diameter but integrates the float chamber within the carburettor body itself, a cleaner design that eliminates the external float bowl and its associated pipework. The HIF4 was used in several different specifications through to the end of production, with changes to the needle type, slow run kit, throttle disc, and mixture screw configuration occurring between early chrome bumper, rubber bumper, and late tamper-proof versions. On HIF4 carburettors, earlier specifications used a plain throttle disc while later types incorporated an overrun valve disc, and the rebuild kits differ accordingly.
These internal differences mean that service and rebuild components must be ordered to match the specific carburettor specification, not simply the carburettor type.
Needles & Tuning
The needle is the single most important tuning component in an SU carburettor. Its tapered profile controls the fuel mixture at every point in the throttle range, and the correct needle specification varies with the engine type, carburettor specification, and any modifications to the engine's breathing. Standard needle specifications are documented for each carburettor type across production, but modified engines, particularly those with performance camshafts, ported cylinder heads, or high-flow air filters, will likely require a different needle profile. Ideally, modified cars should be set up on a rolling road to determine the optimal needle.
Kits & Individual Components
Gasket kits cover the essential seals and washers for routine carburettor maintenance. Service kits extend this to include float chamber needles, seats, and jet assemblies. Full rebuild kits add throttle spindles, bushes, and throttle discs for a comprehensive overhaul, noting that the throttle spindle bosses on the carburettor body must be reamed when fitting new bushes. Sundries kits provide the smaller hardware items such as screws, springs, and lock tabs.
Individual components including dampers, dashpot piston springs, jets, float assemblies, throttle return and choke return springs, bi-metal assemblies, and mixture screws are all available separately. Dampers come in the original black nylon top or a bright brass-finished alternative. Dedicated SU tuning tools and carburettor balancing equipment are also available to assist with setup and adjustment.