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  Chicagoland MG Club:Club Info
Repairing SU Fuel Pumps, Type L
by Bob Tait, Australia

Dismantling the pump:
1. Remove the Bakelite cap by removing the cap nut and lower nuts from the stud assembly.
2. Remove the 6 screws holding the coil housing to the pump body.
3. Carefully separate the diaphragm from the pump body taking care not to damage the material.
4. Split the pump body apart by inserting a thin screw driver into the gasket, open with a slight twist.
5. Unscrew the outlet fitting from the pump body, then lift out the outlet valve assembly taking care not to lose the inlet valve. Remove the circlip to release the outlet valve.
6. Undo the filter nut in the pump body and remove the filter.
7. Unscrew the diaphragm assembly taking care not to lose the support rollers.
8. Remove the upper contact from the pedestal.
9. Using a pair of needle nose pliers twist the pin holding the rocker assembly to the legs of the pedestal then gently withdraw it to release the rocker assembly.
10. Remove the two pedestal screws taking care not to break the actuating coil wire.
This completes dismantling of the pump.

Inspecting and testing
Test the resistance of the coil which should read 6 ohms. The coil can be tested for current draw which should read 2A. A higher current would indicate shorted turns, a lower reading or no reading an open circuit or high resistance connection.
Inspect the contact on the rocker assembly and overhead contact; they should be clean and bright. If oxidized they can be refurbished using 600 grit carborundum paper.

After cleaning burnish the contact with a worn contact file until a mirror finish is obtained. The contact surfaces should be slightly convex.
Measure the 11 rollers for ovality and size. They should be round and between 0.427 and 0.431 inch diameter. A 0-1" micrometer is ideal for this task.
Check the diaphragm for cracking, flaking and hardness; it should be supple to feel. The outer edges should not be frayed. Check the spindle for run out and alignment. Replace if worn or damaged.
Check the pump body components for flatness of mating surfaces, slight distortion can be rectified by resurfacing using a surface plate and emery sheet at least 400 grit.
Inspect the brass valves for wear on the seating surface, some valves are stippled on the upper surface. The valves can be resurfaced by rubbing on 600 grit emery on a surface plate, try to do this in at least two different directions to improve flatness.
The gasket between the pump body and the sandwich plate should always be replaced.
Check all washers for cracks and damage, replace as necessary.
Check the drain hole in the coil housing is clear.

Lapping valves
First is the inlet valve, place about 3 to 4 drops of Brasso into the pump body then place the inlet valve stipple side upwards. Using a sharp scriber, place this in the center of the valve. Using a circular motion which will rotate the valve on its seat lap the valve until all the lower surface is covered in small orbital rings in appearance.
Wipe out the Brasso, then holding a finger over the two holes in the body blow downwards against the valve with the mouth. No loss of pressure should be observed, if it leaks continue the process. The outlet valve is lapped in a similar way except that it is installed into the brass outlet carrier. When finished install the circlip to retain the valve.
SU fuel pump Type-L explosin diagram
Once the valves are lapped in they are ready to be reinstalled into the pump body.
First place the inlet valve then the thin fibre washer, next install the outlet valve carrier then the slightly thicker fibre washer. Then fit the outlet union and tighten.
Next fit the new cork gasket between the pump body and sandwich plate ensuring the holes align with the slot in the sandwich plate.
Next carefully fit the rocker assembly to the pedestal using the fulcrum pin through the legs of the pedestal. Install the two holding down screws fitting the rocker earth wire under the appropriate screw. Next install the upper contact and the coil terminal on top, tighten the screw once the contacts are aligned.
Next install the diaphragm and spring ensuring that the spindle is engaged correctly into the rocker assembly then screw clockwise until the flange on the spindle is level with the first step in the coil housing. Refit the 11 rollers, with the diaphragm uppermost press downward on the diaphragm the rocker should toggle and open the contacts. To adjust the diaphragm turn the diaphragm clockwise until the rocker just does not toggle, then back off one complete turn or six holes anticlockwise.
When pressing on the diaphragm it should now toggle very easily. This completes the adjustment.
Place the coil housing on the assembled pump housing ensuring the drain hole aligns with the filter cap; insert 6 screws until they are just touching the coil housing flange. Now place a small screwdriver under the pedestal to lift the inner section of the rocker (the one attached to the diaphragm) lift upward then tighten all 6 screws in opposing sequence evenly. This ensures the diaphragm has free movement.
If the contacts are correctly adjusted, pressing the upper contact against the pedestal should provide equal distance between the fibre rollers of the rocker and the top of the coil housing and the upper brass stirrup of the rocker and the underside of the Bakelite pedestal.
The suppression capacitor across the points is .01 MFd rated at 250 volts, it should be a Styroseal type if possible. (Not polarized).

Testing your "L" pump
When testing your pump you need a small tank with an outlet pipe at the bottom and an inlet pipe at the top. A capacitor of 4-5 litres should be adequate. It is recommended to use kerosene as it is less volatile than petrol.
Mount the pump approximately 42" above the tank with the filter cap at the bottom and the outlet at the top. Connect the inlet of the pump to the tank outlet. Then connect the pump outlet via a pressure gauge then a stop tap then a return line to the tank. The tap is used for checking the pump's ability to hold pressure.
Next we need a power supply capable of supplying 12 volts at about 4 amps.
We are now ready to start the test.
Connect the 12 volt supply to the pump and allow time for the pump to prime. You should have free flow with no air bubbles. Now slowly close the tap until it is fully closed. The pressure should rise from 0 to 1.5 PSI or 10KPa; with tap fully closed the pump should only tick about once per minute. Anything less than this indicates the valves are leaking and more lapping is required. If the pressure is low the diaphragm spring is below tension. Often a slight stretch of the spring can fix this.
- The minimum deliver rate is 8 gallons/hour or 36 litres/hour
- The pipe bore for the test stand is 1/4" pipe
- Pressure gauge 0 to 5 PSI
- Stop tap 1/4" fittings
- 14" clear hose, petrol resistant.

Bob Tait

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