The MGC uses a progressive linkage design connecting the twin SU HS6 carburettors both to each other and to the throttle and choke control cables. This linkage arrangement is considerably more complex than the equivalent system on the four-cylinder MGB, owing to the greater physical distance between the two carburettors on the longer six-cylinder engine and the need for a progressive throttle action that opens the carburettors sequentially for smooth power delivery. The front carburettor opens first from idle, with the rear carburettor joining progressively as the throttle is opened further, this sequential action provides smoother low-speed response and more predictable power delivery than a simple simultaneous arrangement would on an engine of this capacity. The progressive linkage was a sophisticated piece of engineering for its era, but it was fitted to relatively few cars, fewer than 9,000 in total, and this low production volume has had a direct impact on parts availability today.
How the Linkage Works
The throttle cable from the pedal connects to a relay shaft bracket on the front carburettor, which drives a connecting rod running rearwards to the rear carburettor. The progressive mechanism is built into the front lever assembly, incorporating a cam and follower arrangement that controls the rate at which the rear carburettor opens relative to the front. The choke mechanism works in a similar fashion, with a choke cable from the dashboard driving both carburettor choke mechanisms through a separate set of levers and connecting rods. The linkage geometry must be correctly set up for the progressive action to work as intended, if the levers are worn, the pivot points are sloppy, or the connecting rod is bent, the two carburettors will not open in the correct relationship, resulting in uneven fuelling between the two banks of cylinders and poor throttle response across the rev range.
Parts Availability
Very little of the original progressive linkage assembly is available as new manufacture. The front and rear lever assemblies from the throttle and choke connecting rods are the principal items available, along with carburettor return springs. These lever assemblies are the critical wearing parts in the linkage, transmitting the driver's throttle and choke inputs to the carburettor mechanisms. When these levers become worn or sloppy, the result is an imprecise throttle response and inconsistent idle, as the two carburettors no longer open and close in the correct synchronised relationship.
Owners requiring other 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 or serviceable used units.
Throttle and Choke Cables
The throttle cable and choke cable route from the bulkhead through the engine bay to the carburettor linkage. The throttle cable connects to the accelerator pedal inside the car via a pivot mechanism, while the choke cable connects to the dashboard-mounted choke control. Both cables are subject to fraying, stiffening, and eventual breakage, a stiff choke cable is a particular nuisance, as it prevents the choke from returning fully when pushed home, leaving the mixture over-rich and causing poor fuel economy, plug fouling, and bore wash on the cylinder walls. The throttle and choke cables themselves are shared with the contemporary MGB (1967 to 1969 specification, chassis numbers 138401 to 187210), but the carburettor linkage that these cables connect to, the mechanism that synchronises the two carburettor throttle and choke actions, is entirely MGC-specific.
Carburettor Return Springs
Return springs ensure the throttle butterflies close fully when the accelerator pedal is released. Weak or broken return springs are a safety concern, if the throttle does not snap closed when the pedal is lifted, the engine will continue to race, with potentially dangerous consequences. Springs should be inspected regularly for stretching, corrosion, or loss of tension, and replaced at the first sign of any deterioration. The correct spring rate matters: too light and the throttle may not close reliably, too heavy and the pedal feels unnecessarily stiff and tiring on long journeys.