The MGC uses a progressive linkage design connecting the twin carburettors to each other and to the throttle and choke control cables. This arrangement is considerably more complex than the MGB equivalent, owing to the greater physical distance between the two carburettors on the longer inline six-cylinder engine. The progressive action ensures the carburettors open sequentially for smooth power delivery, the front carburettor begins to open first from idle, with the rear carburettor joining progressively as the throttle is opened further. This sequential action avoids the sudden fuel enrichment that would occur if both carburettors opened simultaneously, providing smoother and more controllable power delivery particularly at low speeds and during gentle throttle openings in traffic.
Available Components
Very little of the original progressive linkage assembly remains available as new manufacture, a direct consequence of the design complexity and the small production numbers. Fewer than 9,000 MGCs were built, and the progressive linkage was unique to the model, meaning there was never a large enough aftermarket demand to justify tooling for reproduction parts. The front and rear lever assemblies from the throttle and choke connecting rods are the principal items available, these are the most common wearing parts, as the pivot points gradually develop play that introduces sloppiness into the linkage action. When these levers are worn, the two carburettors no longer open and close in the precise synchronised relationship that the progressive mechanism requires, resulting in uneven fuelling between the front and rear three cylinders and a throttle response that feels imprecise and inconsistent.
Carburettor return springs (two required) pull the carburettors back to idle when the throttle pedal is released, weak or broken springs are a safety concern and should be replaced at the first sign of stiffness or loss of tension.
Carburettor Mounting Hardware
Manifold and insulator gaskets (six required), carburettor-to-manifold studs (eight), nuts with spring and plain washers, and insulating blocks (two) complete the carburettor mounting hardware. The insulating blocks sit between the inlet manifold and the carburettor flanges, reducing heat transfer from the hot manifold to the carburettor bodies, this insulation is important because the SU carburettor's fuel metering is sensitive to temperature, and excessive heat causes fuel to vaporise in the float chambers and jets, resulting in rich running, hot-start difficulties, and poor fuel economy. All six gaskets should be renewed whenever the carburettors are removed from the manifold, reusing old gaskets risks air leaks that introduce unmeasured air into the inlet tract, weakening the mixture in ways that carburettor adjustment cannot correct.
Sourcing Scarce Components
Owners requiring other progressive linkage components, connecting rods, pivot bushes, cable brackets, or the progressive mechanism itself, should contact MGOC Spares, as individual items may be sourced on a case-by-case basis from specialist suppliers, from other MGC owners via the club network, or from serviceable used units. Given the rarity of these components, retaining all original linkage parts during any engine or carburettor overhaul is strongly advised, even worn items have value as cores for reconditioning or as patterns for reproduction.