The MGB was fitted with SU carburettors throughout its production life, and the carburettor type changed several times across the model range. Understanding which type is fitted to the engine is essential when ordering replacement carburettors, service components, needles, or rebuild kits, as the internal parts differ between types and are not always interchangeable.
HS4 Carburettors (1962 to 1973)
The original carburettor was the SU HS4, a horizontal semi-downdraught type with a 1½-inch choke diameter. These were fitted as a pair from the start of production in 1962, and were used on all home market cars through to late 1973.
Early HS4 carburettors used fixed needles, the needle profile determines the fuel mixture across the rev range, and the correct needle specification varies with the engine type and carburettor specification fitted. From the introduction of the 18V-series engine in 1971, home market HS4 carburettors adopted spring-loaded biased needles in place of the earlier fixed type.
This is an important distinction when ordering replacement needles, needle locking screws, and jet bearing kits, as the internal components differ between fixed and biased needle carburettors.
HIF4 Carburettors (1973 to 1980)
The SU HIF4 (Horizontal Integral Float) carburettor replaced the HS4 on export cars from 1972 and on home market cars from November 1973. The HIF4 shares the same 1½-inch choke diameter as the HS4 but features an integral float chamber rather than the separate external float bowl of the HS4. The HIF4 was used through to the end of production in 1980 on all UK and non-North American export models, though the carburettor specification and needle type changed several times, notably when the rubber bumper models were introduced in late 1974, and again when tamper-proof carburettors with shorter bellhousing necks were introduced around the 1977 model year. The internal components of the HIF4, including throttle disc type, slow run kit, and jet specification, vary between these different carburettor specifications, so identifying the correct specification is important when ordering service or rebuild parts.
Gasket kits, service kits, and full rebuild kits are available for both HS4 and HIF4 carburettors. Gasket kits cover the essential seals and washers needed for routine maintenance. Service kits add float chamber needles, seats, and jet assemblies. Full rebuild kits further include throttle spindles, bushes, and throttle discs, noting that on HIF4 carburettors, earlier specifications used a plain throttle disc while later types used a disc incorporating an overrun valve.
When fitting throttle spindle bushes, the throttle spindle bosses on the carburettor body must be reamed to ensure correct fit. Sundries kits covering the smaller hardware items, screws, springs, lock tabs, and float pivot pins, are also available for both types. Individual components including dampers, dashpot piston springs, jets, float assemblies, throttle return springs, choke return springs, and mixture screws are available separately for owners carrying out targeted repairs rather than a complete overhaul. Dampers are available in the original black nylon top or in a bright brass-finished alternative.
Performance & Upgrade Options
For performance-oriented owners, the larger 1¾-inch HS6 carburettor conversion offers improved torque at low to mid revs over the standard 1½-inch setup. Gasket kits, service kits, and rebuild kits for the HS6 are also available. A Weber 45 DCOE twin choke conversion is offered as an alternative for those seeking more significant performance gains. When working on SU carburettors, correct needle selection is critical to achieving the right fuel mixture.
Modified cars, particularly those with performance camshafts, cylinder head work, or high-flow air filters, should ideally be set up on a rolling road to determine the optimal needle specification. The MGOC Spares range encompasses needles for all HS4, HIF4, and HS6 applications, together with dedicated SU tuning tools and carburettor balancing equipment to assist with setup and adjustment.