MGB Workshop Manual p134 - SU Carburetter Removal, Tuning & Float Level (D.4)
MGB carburetter service procedure: checking nylon float level (Fig D.11), removing/refitting twin carburetters and slow-running mixture/throttle synchronisation, non-emission cars.
This is page 134 (page D.12) from the MGB workshop manual, Section D — The Fuel System, covering SU carburetter service. It begins with Fig. D.11, showing how to check the nylon float level using a 1/8 to 3/16 in. (3.18 to 4.76 mm) diameter bar across the machined lip of the float chamber, with a legend identifying the float lever resetting point, needle valve assembly, and hinge pin. Section D.4 (Carburetters) opens with an important note that the adjusting, dismantling, and reassembly instructions apply only to cars WITHOUT exhaust emission control equipment; emission-controlled cars must follow the Engine Emission Control Supplement. The page then details removing and refitting the carburetters (disconnecting fuel supply, overflow pipes, mixture and throttle cables, vacuum ignition pipe, and removing four flange nuts), followed by slow-running adjustments and synchronization: setting jets flush with the bridge, turning the jet adjusting nut down two complete turns (12 flats), balancing throttle screws by listening to intake hiss, and checking mixture strength by lifting the rear carburetter piston 1/32 in. (.75 mm) — speed increase means too rich, speed decrease means too weak. Fig. D.12 shows a feeler gauge placed between the throttle shaft stop and choke interconnecting spindle, with an inset for automatic transmission cars.
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Frequently asked questions
- How do you set the initial jet position when tuning MGB carburetters?
- Remove the pistons and suction chambers, screw the jet adjusting nuts until each jet is flush with the bridge of its carburetter (or as near as possible), refit the pistons and suction chambers, verify the pistons fall freely using the lifting pins, then turn the jet adjusting nut down two complete turns (12 flats).
- How do you check the mixture strength on an MGB carburetter?
- Lift the piston of the rear carburetter by approximately 1/32 in. (.75 mm). If the engine speed increases, the mixture is too rich; if the engine speed immediately decreases, the mixture is too weak.
- How are the throttles synchronized on the twin carburetters?
- Adjust the throttle adjusting screws by equal amounts to get the desired idle, then listen to the hiss in each intake and adjust the screws until the intensity of the hiss is similar on each intake, which synchronizes the throttle setting.
- Do these carburetter instructions apply to emission-controlled cars?
- No. The instructions in Section D.4 apply only to cars not fitted with exhaust emission control equipment. Carburetters on cars with emission control equipment must be tuned per the Engine Emission Control Supplement to the manual.
- What size bar is used to check the nylon float level (Fig. D.11)?
- A bar of 1/8 to 3/16 in. (3.18 to 4.76 mm) diameter placed on the machined lip of the float chamber.
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