Carburettor specification on the MGA splits cleanly between the pushrod cars and the Twin Cam. The pushrod 1500, 1600 and 1600 Mk II share the same basic carburettor architecture with minor needle differences between engine sizes, while the Twin Cam uses a larger, different SU carburettor type altogether, mounted on the opposite side of the engine to the pushrod installations. Throttle and choke linkage routing is correspondingly different between the two, and between right-hand and left-hand drive cars. This node covers complete carburettors, their internal service components, and the throttle and choke cables and linkages that operate them.
Pushrod car carburettors, H.4
All pushrod MGAs use twin SU H.4 semi-downdraught carburettors of 1½ inch diameter, with 0.090 inch jets and hexagonal brass tops. Needle specification changed with engine size. The 1500 was specified with a GS standard needle, with CC available as a richer alternative and No. 4 as a weaker one. The 1600 and 1600 Mk II were specified with a standard No. 6 needle.
When replacing needles, or servicing a carburettor where the correct needle is not confirmed, the car's engine prefix (15GB or 15GD for 1500, 16GA for 1600, 16GC for 1600 Mk II) is the ordering reference. H.4 bodies, throttle spindles, butterfly discs, float chambers, float needle valves, jets, damper tops and jet bearing assemblies are all available as service items. The carburettor balance pipe between the two bodies was finished in engine colour and is an engine-colour item during restoration.
Twin Cam carburettors, HD.6
The Twin Cam uses twin SU HD.6 carburettors of 1¾ inch diameter with 0.100 inch jets. Standard needle specification is OA6, with RH for rich running and OA7 for weak running.
From July 1959 a shorter damper piston was introduced for improved even running and peak speed; these can be identified by the carburettor top caps being stamped 'O'. HD.6 components are not interchangeable with H.4, bodies, jets, needles, float assemblies and damper pistons are larger and specific to the 1¾ inch format. Because the Twin Cam's carburettors also sit on the opposite side of the engine to the pushrod installation, inlet manifold studs, throttle linkage, choke cable routing and air cleaner mounting are all Twin Cam-specific.
Throttle and choke cables
All MGAs have a pendant accelerator pedal with a small square pad carrying vertical grooves, operating the carburettors through a throttle cable. On 1500 cars from chassis 24594 an auxiliary return spring was added to the throttle cable to give more positive return action, a change that carried through the remainder of MGA production.
Replacement throttle cables, outer sheaths, return springs and cable clamps are available as service items. The choke is operated from a dash-mounted pull control marked 'C' through a choke cable to the carburettor jet lever assembly; replacement inner and outer cables, trunnions and jet links are available for both H.4 and HD.6 installations and should be ordered to match the carburettor type.
Accelerator linkage and cross-shaft
On left-hand drive cars, the accelerator pedal operates through a cross-shaft assembly that transfers pedal movement to the throttle cable. The Twin Cam uses a different pattern of cross-shaft from the pushrod cars, and this same Twin Cam-pattern cross-shaft was also fitted to the De Luxe variants.
When ordering cross-shaft components for a left-hand drive car, the combination of drive orientation and engine type (pushrod non-De-Luxe, Twin Cam, or De Luxe) determines the correct part. Right-hand drive cars do not use the cross-shaft in the same form. Bushings, pivots and return springs are available as service items for both arrangements.
Ordering considerations
When ordering carburettor components, the engine prefix is the starting point and determines whether H.4 or HD.6 parts are required, and in the case of the pushrod cars, whether the 1500-specification GS needle or the 1600/Mk II No. 6 needle is appropriate. Owners of cars with later-fitted MGB engines will need to confirm which carburettor pattern is on the car, since MGB pushrod installations used HS-series carburettors not fitted to any factory MGA. Throttle and choke cable lengths vary between RHD and LHD cars, and between pushrod and Twin Cam installations due to the opposite-side carburettor mounting on the Twin Cam, ordering should be to both the car specification and the drive orientation.