The twin SU carburettors are the breathing heart of the XPAG and XPEG. Whether fitted with H2 or H4 instruments, the principle is the same, a variable-venturi design where the air piston rises and falls in response to engine demand, drawing fuel through a tapered needle in a fixed jet. This simplicity makes the SU straightforward to tune and maintain, but bore sizes and detail specifications between the TD, Mark II, and TF mean many components must be matched to the type fitted.
Complete Carburettors
Complete carburettors are available reconditioned on the customer's own unit basis and new outright in all three specifications: AUC549 for the standard TD, AUC578 for the TD Mark II, and AUC728 for the TF. These specifications cannot be mixed, the H2 and H4 carburettors use different bore sizes, and the two H4 variants have different internal configurations appropriate to their respective applications.
Rebuild, Service and Gasket Kits
Three levels of overhaul kit are available, each addressing progressively deeper wear. A gasket kit covers sealing components only, manifold gasket, air filter gasket, suction chamber damper washer, float lid gasket, banjo washers, jet bearing washers and glands, and float chamber hold-up washers. A service kit adds the float chamber needle and seat assembly and the complete jet assembly. A full rebuild kit goes further, adding the throttle spindle and bushes, disc with screws, jet lever and link hardware, tapered pins, and float lever pivot pin, every wear item in the body.
Gasket kits are H2-specific or H4-specific, but service kits and sundries kits are common across both bore sizes. A sundries kit supplements the gasket and service kits with the smaller-value items, gland and jet return springs, pivot bolt, stop screw and suction chamber screws, float levers, banjo bolt, jet sealing ring, locking nut and adjusting spring, float filter, and float lid nut. Worn throttle spindles and body bushes are the commonest cause of erratic idling and difficulty in balancing mixture between the two carburettors. A carburettor in service for decades will benefit from a full rebuild kit rather than a gasket set alone, as worn bushes allow air leakage past the spindle that no mixture adjustment will correct.
Throttle Spindle, Disc and Piston Components
The throttle spindle and disc kit differ between H2 and H4 carburettors, reflecting the bore sizes. The spindle bush for body repair is common across both types and can restore a body where the bore has worn oval. The air piston runs in the suction chamber on the damper shaft; plastic-cap dampers are original with brass-cap upgrades available across all three variants. Piston spring colour: light blue for standard TD and TF, red for the TD Mark II.
Float Chamber Components
Float chambers on each carburettor are separate from the main body, with non-interchangeable front and rear types. The needle and seat assembly controls fuel level.
The original brass needle and seat is supplemented by an improved Grose jet replacement offering better fuel level control and greater resistance to sticking, particularly beneficial with modern ethanol-blended fuels that cause varnish deposits on conventional valves. Float, lever, lid, gasket, cap nut, and alloy washer are available individually. Two lever types are supplied, one for original lids, one for replacement lids, and must be matched to the lid fitted.
Linkage and Interconnection
The throttle linkage interconnecting the two carburettors comprises a connecting shaft, coupling assemblies, jet lever link assembly with fork ends, and various clevis pins, split pins, and washers. The linkage must ensure both carburettors open simultaneously, unequal opening will cause one cylinder pair to run richer than the other, resulting in uneven running and poor economy. Linkage items are common across all three specifications. A carburettor balancer tool is available for equalising airflow during setup, with an SU tuning tool kit for mixture adjustment.
Ordering Considerations
The carburettor specification (standard TD, Mark II, or TF) is the essential reference for ordering. The H2-to-H4 distinction affects gasket kits, throttle spindle, disc kit, piston damper, and manifold spacer. The needle specification is particular to each type and should match the engine's state of tune. Service kits, sundries kits, float chambers, lids, floats, jets, needle and seat assemblies, jet bearings, and most of the interconnecting linkage are common across all three, a useful simplification for any T-series XPAG owner.