The standard SU and Stromberg carburettors fitted to classic MGs were sized for the breathing requirements of the original-specification engines, and any meaningful engine tuning, Stage II cylinder heads, modified camshafts, raised compression ratios, quickly reaches the limit of what the standard carburettor pair can deliver, becoming the limiting factor in performance. Performance carburettor kits cover the upgrade paths available, larger-bore SU conversions for owners staying with the SU family, twin Weber DCOE installations for substantially more breathing capacity, and the supporting hardware needed for either approach, and for owners who have carried out cylinder-head work, fitted a performance camshaft, or upgraded the exhaust system, a carburettor conversion unlocks the full potential of those modifications.
Larger-Bore SU Conversions
The standard MGB used a pair of SU HS4 carburettors with a 1½-inch throttle bore through most of its production, the later cars moving to HIF4 carburettors of the same throttle bore but with integral float chambers, and stepping up to HS6 carburettors with a 1¾-inch throttle bore provides substantially more airflow capacity, typically 30% more peak airflow, while retaining the SU family's friendly tuning behaviour and the visual character that the SU pair brings to the engine bay. The HS6 conversion requires a matched inlet manifold to accept the larger carburettors, supplied as part of the conversion kit, with the linkage renewed to suit the larger flanges and the air-filter housings sized to suit the increased intake demand. The main benefit is a meaningful increase in torque at low to mid-range rpm with a small increase in fuel consumption, the HS6 retaining the SU's characteristic ease of tuning and the availability of a wide range of needle profiles for fine-tuning the mixture, making it a popular choice for fast-road cars where driveability and flexibility are valued alongside outright performance, and the result is an engine that breathes much more freely while looking very similar to the original specification from the engine bay.
Twin Weber DCOE Installations
For owners aiming at more substantial output increases, Stage III and beyond, twin Weber DCOE side-draught carburettors are the established performance alternative, a sophisticated twin-choke side-draught carburettor used widely in classic competition applications with a wide tuning range achieved through interchangeable jets, emulsion tubes, and air-correction circuits. The headline kit is the Weber 45 DCOE installation supplied ready-assembled with a matched inlet manifold, throttle linkages, and a K&N air filter, requiring only a special throttle cable to complete the installation, the twin-choke 45 DCOE giving substantial airflow capacity that easily supports outputs into the 120 to 140 horsepower range on a properly-tuned B-Series. The Weber was referenced in the Abingdon Special Tuning Department's own tuning stages during the production life of the cars, giving it a genuine competition heritage, and it is particularly effective on engines already built to a fast-road or competition specification. The trade-off compared with the SU pair is more involved initial setup and a more aggressive throttle response, and equivalent twin-DCOE kits cover the other applications with matched manifold, linkages, fuel-supply hardware, and air-filter arrangement included.
Supporting Hardware & Setup
Any performance carburettor conversion benefits from supporting changes alongside the headline carburettor change. Performance fuel pumps are typically needed to keep up with the increased petrol demand, a standard SU pump being adequate for an HS6 conversion on a mildly modified engine but more heavily tuned engines and particularly those with a Weber benefiting from a performance pump and a fuel-pressure regulator to bring the higher pump pressure back to the carburettor's design specification and ensure consistent flow. The ignition system should be reviewed, as the standard distributor curve may not suit the engine's revised airflow characteristic, and a re-curved distributor or full electronic ignition may produce meaningful additional gains, while the exhaust side benefits from a matching upgrade, a tubular manifold and free-flow system allowing the engine to exhale at the rate the new carburettors allow it to breathe in. With either conversion, carburettor needle selection is critical, the standard needles potentially needing changing to suit the individual engine specification particularly where other breathing modifications have been made, and modified cars should ideally be set up on a rolling road after installation to optimise the fuel mixture across the rev range. Suitable air filters, heat shields, and stub stacks for both conversions are available in their respective sections, and the technical team is available to advise on the right combination for a specific application.