The SU carburettors fitted to every MGA were designed to be serviced and rebuilt rather than replaced. Service kits cover the wearing consumable parts, gaskets, needle valves, jet washers, float needles and associated small fittings, that should be renewed as part of routine maintenance or when a carburettor is being disturbed for cleaning. Rebuild kits cover the additional items that wear only over extended service, principally the throttle spindle bushes and jet bearing assembly, and are used when a carburettor has been in service long enough to show air leaks at the throttle spindles or jet lift inconsistency. Because the MGA uses two distinct carburettor types, the correct kit type depends on the engine family fitted to the car.
Pushrod service and rebuild kits, H.4
The 1500, 1600 and 1600 Mk II all run twin SU H.4 1½ inch semi-downdraught carburettors. Service kits for the H.4 cover the items that should be renewed during routine maintenance: float chamber gasket, float needle valve, jet seal washers, top cover gaskets, damper cap sealing washers and the small retaining hardware. Rebuild kits extend this to include throttle spindle bushes, the most common cause of an air leak on a long-serving SU, and the jet bearing and gland components. Because twin-carburettor MGAs need both bodies serviced together, kits are available in both single-carburettor and paired form, and the paired quantity is the correct starting point for any car running as the factory specified.
Needles and jets are not normally included in service or rebuild kits and are ordered separately. The 1500 uses a GS needle as standard; the 1600 and 1600 Mk II use a standard No. 6 needle. The 0.090 inch jet is common to all pushrod MGAs.
Twin Cam service and rebuild kits, HD.6
The Twin Cam's twin SU HD.6 1¾ inch carburettors require a different kit specification. Float chamber dimensions, jet diameter (0.100 inch rather than 0.090 inch), needle profile, damper piston length and throttle spindle size are all different from the H.4. HD.6 service and rebuild kits are supplied specifically to the Twin Cam carburettor type and are not interchangeable with H.4 kits.
From July 1959 the Twin Cam received a shorter damper piston to improve running, identifiable by the top caps being stamped 'O'; where a rebuild kit includes a damper piston, the kit should be matched to the pre- or post-July-1959 specification of the car. HD.6 needles (OA6 standard, RH rich, OA7 weak) are ordered separately from the rebuild kit. The 0.100 inch jet is specific to the HD.6.
What is and is not a kit item
In normal service, the carburettor bodies, top covers, piston and dashpot assemblies, and the throttle butterfly discs are reusable and are not kit items. A service kit is therefore intended to refresh the sealing and float-side wearing parts, while a rebuild kit addresses the mechanical wear items that develop only over long service, principally throttle spindle clearance.
Where a carburettor shows major body damage, jet bore wear beyond the jet bearing adjustment range, or a distorted top cover, individual replacement components are ordered separately rather than as part of a kit.
Ordering considerations
The engine prefix determines whether H.4 or HD.6 kits are required. Cars that have been re-engined with an MGB pushrod unit will usually be running HS-series carburettors rather than H.4, and their kits are not part of the MGA catalogue. Owners ordering for a twin-carburettor installation should normally order a paired quantity so both bodies can be serviced in matching condition.
Where only one carburettor is being disturbed, a single-carburettor kit quantity is correct but the companion body should be inspected and tuned afterwards to ensure the pair balance correctly.