MGB Twin SU Carburetter Tuning - Initial Settings & Tuning Conditions (Page 428)
Service procedure page covering twin-carburetter initial settings, tuning conditions and adjustment steps with labeled figures.
This is page 428 (section 4-12, 'Engine Emission, Issue 3, 24062') of a British Leyland-style workshop manual covering twin SU carburetters, uploaded to a 1971 MG B gallery. The page shows a labeled illustration 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. It gives the 'Initial settings' procedure: slacken the throttle spindle interconnection clamping bolts, disconnect the jet control interconnection and mixture control wire, back off the fast-idle and throttle adjusting screws until clear, set throttle screws half a turn open, and do not alter the jet adjusting nuts (which are factory datum-set). 'Tuning conditions' describe the setting cycle: connect a tachometer and exhaust gas analyser, warm the engine to operating temperature in an ambient of 16–27° C (60–80° F), run at least five minutes after thermostat opening, run at 2,500 rpm at no load for one minute, and complete tuning within three-minute intervals, clearing the engine at 2,500 rpm between periods. The 'Tuning procedure' heading begins with a cutaway carburetter illustration; the procedure text continues on the next page. Illustration reference numbers D0474A, A6414B, A6401C, and A6395A appear.
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Frequently asked questions
- What are the initial settings for twin carburetters before tuning?
- Slacken both clamping bolts (10) on the throttle spindle interconnections, disconnect the jet control interconnection by slackening clamping bolts (11), disconnect the mixture control wire if fitted, unscrew the fast-idle screws until well clear of the cams, unscrew the throttle adjusting screws until just clear of their stops with throttles closed, then set the throttle adjustment screws half a turn open. Do not alter the jet adjusting nuts at this stage.
- Why shouldn't the jet adjusting nuts be altered during initial settings?
- Because they will be initially set to a datum setting either at the factory or during the carburetter servicing procedure.
- How should the engine be warmed up before carburetter tuning?
- Warm the engine at a fast idle to normal operating temperature, preferably in an ambient temperature of 16–27° C (60–80° F), and run it for at least five minutes after the thermostat has opened; then run at 2,500 rpm at no load for one minute.
- What if carburetter tuning takes longer than three minutes?
- If the settings exceed a three-minute period, open the throttle and run the engine at 2,500 rpm for one minute, then resume tuning. Repeat this clearing operation each time a further three-minute period is exceeded.
- What do the numbered callouts on the twin-carburetter installation diagram identify?
- 1 = jet adjusting nuts, 2 = jet locking nuts, 3 = piston/suction chambers, 4 = fast-idle adjusting screws, 5 = throttle adjusting screws, 7 = jet adjustment restrictors.
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