MGB Workshop Manual p.131 - SU HS4 Carburetter Diagrams & Fuel Pump Faults
MGB fuel system page: sectional and exploded views of the SU H154 fuel pump (jet locking nut, throttle stop screw, float-chamber, piston) plus diaphragm/noise fault diagnosis.
This is page 131 (section D.9) from an MGB workshop manual chapter titled 'The Fuel System'. The page covers SU fuel pump troubleshooting and includes two labeled illustrations of the SU HS4 carburetter. Text sections address: (iv) faulty diaphragm action — advising removal of the coil housing, flexing the diaphragm without losing the 11 rollers underneath, and lightly oiling the throw-over spring spindles; (2) noisy pump — diagnosing air leaks in the suction line by submerging the discharge pipe end and watching for continuous bubbles, then checking tank-to-pump connections, the inlet union 'O' ring, and coil housing screws; and (3) pump operating without delivering fuel — caused by serious suction-side air leaks or foreign matter under the valves, with a caution not to scratch the plastic valve disc. Fig. D.9 is an external exploded-style view of the HS4 carburetter with an 8-item legend (jet adjusting nut, throttle stop screw, choke/fast-idle screw, jet locking nut, float-chamber securing nut, jet link, jet head, vacuum ignition take-off). Fig. D.10 is a sectional view with a 7-item legend (jet locking nut, jet adjusting nut, jet head, feed tube from float-chamber, piston lifting pin, needle securing screw, oil damper reservoir). Footer reads 'MGB. Issue 5. 69615'.
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Frequently asked questions
- What do the numbered parts in the HS4 carburetter sectional view (Fig. D.10) represent?
- 1. Jet locking nut; 2. Jet adjusting nut; 3. Jet head; 4. Feed tube from float-chamber; 5. Piston lifting pin; 6. Needle securing screw; 7. Oil damper reservoir.
- What causes a noisy SU fuel pump on an MGB?
- An air leak at the suction line. It can be checked by disconnecting the fuel pipe from the carburetter and letting the pump discharge into a container with the pipe end submerged — continuous bubbles confirm an air leak.
- Why does the fuel pump operate without delivering fuel?
- The most likely causes are a very serious air leak on the suction side, or foreign matter lodged under one of the valves, particularly the inlet valve.
- Can compressed air be blown through the fuel pump to clear a blockage?
- No. Compressed air may be used to clear the line to the tank after removing the filler cap, but it should not be passed through the pump as this will cause serious damage to the valves.
- How is faulty diaphragm action remedied?
- Remove the coil housing and flex the diaphragm a few times, taking care not to lose any of the 11 rollers under it. Before resetting, apply a little thin oil very sparingly to the throw-over spring spindles where they pivot in the brass rockers, then reassemble the diaphragm/armature assembly.
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