How do I check the carburetter piston with the lifting pin?

With the engine stopped, check that the piston falls freely onto the bridge, indicated by a distinct metallic click, when the lifting pin (6) is released. If not, refer to 'CARBURETTER SERVICING'.

How much free play should the mixture control wire have?

Adjust the mixture control wire (8) to give a free movement of approximately 1/16-in. before it starts to pull on the jet lever (9).

Can the jet adjustment restrictor be removed on twin carburetters?

No. In no case should the jet adjustment restrictor be removed or repositioned. Only mixture adjustments within the limits of the restrictor are available for tuning.

How should twin carburetters be balanced?

Balancing of twin carburetters must only be carried out with the use of an approved balancing meter. If satisfactory adjustment or balancing is not obtainable within the restrictor limits, refer to 'CARBURETTER SERVICING'.

To what level should the piston damper be topped up?

Top up the piston damper with the recommended engine oil until the level is 1/2-in. above the top of the hollow piston rod.

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MG B SU Carburetter Tuning & Twin Carburetter Adjustment - Engine Emission p419

Carburetter tuning instructions with numbered steps and twin-carburetter figures

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Service procedure page for tuning twin carburetters with step-by-step jet and throttle adjustment instructions and labeled figures.

AI caption September 2012 · Photo 61 of 101

This is page 419 (section 4-3) of an 'Engine Emission' service manual (Issue 3, 12383) covering carburetter tuning procedures, uploaded to a 1971 MG B gallery. The page continues a single-carburetter tuning procedure: step (4) checks that the piston falls freely onto the bridge with a metallic click when the lifting pin is released; step (5) covers turning the jet adjusting nut within the restrictor limits for maximum smooth-running speed; step (6) readjusts the throttle adjusting screw for correct idle; steps (7) and (8) cover mixture control wire free play (approx. 1/16-in.) and fast-idle screw adjustment per 'TUNING DATA'. A 'TWIN CARBURETTERS' section shows a labeled illustration (DO474A) of a twin-carburetter installation identifying jet adjusting nuts, jet locking nuts, piston/suction chambers, fast-idle adjusting screws, throttle adjusting screws, and jet adjustment restrictors. A NOTE warns that the jet adjustment restrictor must never be removed or repositioned and that balancing must be done only with an approved balancing meter. The final step instructs topping up the piston damper with recommended engine oil to 1/2-in. above the top of the hollow piston rod. Illustration reference numbers A6335B, A6401B, A.6394B, DO474A, and A6395A appear.

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Frequently asked questions

How do I check the carburetter piston with the lifting pin?: With the engine stopped, check that the piston falls freely onto the bridge, indicated by a distinct metallic click, when the lifting pin (6) is released. If not, refer to 'CARBURETTER SERVICING'.
How much free play should the mixture control wire have?: Adjust the mixture control wire (8) to give a free movement of approximately 1/16-in. before it starts to pull on the jet lever (9).
Can the jet adjustment restrictor be removed on twin carburetters?: No. In no case should the jet adjustment restrictor be removed or repositioned. Only mixture adjustments within the limits of the restrictor are available for tuning.
How should twin carburetters be balanced?: Balancing of twin carburetters must only be carried out with the use of an approved balancing meter. If satisfactory adjustment or balancing is not obtainable within the restrictor limits, refer to 'CARBURETTER SERVICING'.
To what level should the piston damper be topped up?: Top up the piston damper with the recommended engine oil until the level is 1/2-in. above the top of the hollow piston rod.

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