MGB Twin SU Carburetter Tuning & Balancing Procedure - Engine Emission Manual
Service procedure page detailing carburetter throttle and idle adjustment steps with accompanying labeled figures.
This is page 420 (section 4-4) of a British Leyland engine emission workshop manual section titled 'CARBURETTERS', covering the tuning and balancing procedure for twin SU carburetters. A note explains that on dust-proofed carburetters (identified by a transverse hole in the neck of the suction chambers and no vent hole in the damper cap), the damper oil level must be ½ in. below the top of the hollow piston rod. Steps (2) through (11) are covered: checking throttle action for sticking, checking idle speed against 'TUNING DATA', removing air cleaners, slackening clamping bolts (10) on the throttle spindle interconnections, disconnecting the jet control interconnection at clamping bolts (11), restarting and setting idle with the throttle adjusting screws, balancing the carburetters with an approved balancing meter, adjusting the jet adjusting nut (1) for maximum smooth idle speed, readjusting throttle screws (5) equally, and setting the throttle interconnection clamping levers so the link pin is .012 in. from the lower edge of the fork with approximately 1/32 in. end-float on the interconnection rod. Three illustrations (A6414B, DO473 — a balancing meter in use, and A6366E/A6382A) identify the linkage components. Troubleshooting notes direct the reader to check intake leaks (brake servos, emission equipment) or 'CARBURETTER SERVICING' if balance or idle cannot be achieved.
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Frequently asked questions
- How do you identify dust-proofed carburetters and what is their oil level?
- Dust-proofed carburetters are identified by a transverse hole drilled in the neck of the suction chambers and no vent hole in the damper cap; the oil level must be ½ in. below the top of the hollow piston rod.
- What clearance should the throttle interconnection link pin have from the fork?
- Set the throttle interconnection clamping levers (10) so that the link pin is .012 in. away from the lower edge of the fork, and tighten the clamp bolts ensuring approximately 1/32 in. end-float on the interconnection rod.
- What should you do if the carburetters cannot be balanced correctly?
- Check the intake system for leaks (e.g. brake servos, engine emission control equipment); if still unsatisfactory, refer to 'CARBURETTER SERVICING'.
- How is the jet adjusting nut set during tuning?
- Turn the jet adjusting nut (1) on both carburetters through the full range of adjustment available within the limits of the restrictor, selecting the setting where maximum speed is recorded on the tachometer consistent with smooth running.
- How are the carburetters balanced?
- Using an approved balancing meter in accordance with the maker's instructions, balance the carburetters by altering the throttle adjusting screws; the idling speed obtained must be as given in 'TUNING DATA'.
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