The needle and jet are the heart of the SU carburettor’s fuel metering system. As the piston rises under engine vacuum, the tapered needle is drawn upward through the jet orifice, progressively increasing the fuel flow area. The profile of the needle, its diameter at each point along its length, determines the fuel mixture at every stage of the throttle range, from idle through cruising to full power. Selecting the correct needle for the engine specification is therefore one of the most critical aspects of SU carburettor tuning.
Two fundamentally different needle types were used across MGB production. Early HS4 carburettors from 1962 through to 1971 used fixed needles, which are held rigidly in the piston by a locking screw. The needle profile was changed several times as the engine and carburettor specifications evolved, the correct standard needle varies with the specific carburettor specification fitted, and richer or weaker alternatives were quoted by the factory for each specification.
From 1971, home market HS4 carburettors on 18V-series engines adopted spring-loaded biased needles, which are mounted on a spring and guide that allow the needle to self-centre within the jet. This arrangement compensates for minor misalignment between the piston and jet, improving consistency and reducing wear. HIF4 carburettors, fitted from 1972 onwards, also use biased needles throughout. The distinction between fixed and biased needle types is important when ordering, as the internal components differ.
Fixed needle carburettors use a simple locking screw, while biased needle carburettors require a needle guide and bias spring in addition to the locking screw, and the locking screw itself is different between the two types. Fitting a biased needle into a fixed needle carburettor, or vice versa, without the correct associated components will result in incorrect fuel metering.
Jets & Jet Bearings
The jet assembly sits in the base of the carburettor body and provides the fuel orifice through which the needle operates. Jets differ between the front and rear carburettors on HS4 installations, and between HS4 and HIF4 types. The jet bearing kit, which includes the bearing, seal, and associated washers, also differs between fixed and biased needle HS4 carburettors, and again between HS4 and HIF4 types. A worn jet bearing will allow fuel to weep past the jet, causing a persistent rich mixture that cannot be corrected by needle selection alone.
On modified engines, particularly those with performance camshafts, ported cylinder heads, high-flow air filters, or exhaust upgrades, the standard needle profile may no longer provide the correct mixture across the rev range. In these circumstances, a richer or different-profile needle may be needed. The correct needle for a modified engine cannot reliably be determined from a chart alone, the car should ideally be set up on a rolling road where the mixture can be measured and optimised under load at each point in the throttle range. Needles are identified by a number or combination of letters stamped on the shank, and are always sold individually, two are required per car.
The MGOC Spares range covers the full selection of standard, rich, and weak needle profiles for all HS4 and HIF4 carburettor specifications, together with jet assemblies, jet bearing kits, needle guides, and bias springs.