Fuel system & electrical equipment
General specifications - Fuel supply
The petrol tank is fitted to the rear of the chassis and petrol is fed to the carburettor by an "Autovac" under the bonnet. Capacity 8 gallons.
The S.U. Petrolift draws petrol from the rear tank to its own float chamber, from where it is gravity fed to the carb float chamber. The action of the pump is as follows:
1) A current passing through the solenoid via the contacts, returns to the battery via the chassis. Energising the solenoid draws up the plunger, at the top of the plunger is a flat disc value held in position by a split spring centre tube, this results in fuel being drawn from the tank into the bowl through the inlet and up through the foot value.
2) When the plunger rises it comes up against the sleeve which can be lifted. The contact points will open, the solenoid becomes de-energised and the plunger drops down. As it drops, the lower valve flap is closed by the weight of the fuel and the upper value is lifted upwards, the fuel flowing upwards to the float chamber.
3) The opening of the contacts is governed by a brass sleeve with a steel brush fitted at the top and attached to a cork float. On either side of the central tube is a horseshoe magnet. The moving contact is fitted to an iron plate which pivots in the centre. The upper contact is fitted in a spring plate insulated from the body, but in contact with the solenoid. When the sleeve is at its lowest position the steel bush is bringing the magnets and shunting the magnetic lines of force directly across from one magnet to the other. The upper poles of the magnet are open and exert a pull on the upper portion of the iron contact plate, pulling it towards the magnets so the opposite end carrying the contact is tilted and the contacts close to allow the contact to pass through the solenoid. The energised solenoid draws up the plunger which lifts the sleeve. This means the steel bush lifts bridging the upper poles of the magnets. The magnetic pull on the contact plate is reversed and the lower portion of the iron contact plate drawn towards the magnet, opening the contacts, the solenoid is de-energised and the plunger drops back to its original position and the sequence of events starts over again.
The float chamber. As the chamber fills the cork float rises and a hairpin restricts the fall of the sleeve so that as long as the float chamber remains full the upper poles of the magnet will be bridged so the contact points will be held apart and the plunger ceases working.
The construction is simple and should it cease to function the following should be looked at:
First remove the top cap (22) from the pump to see if the float chamber contains petrol. If it does then the trouble is not with the pump. If it continues to make a pumping noise without delivering petrol it is due to lack of petrol in the back tank, an air leak, or the foot value (6) is held up.
An air leak may be due to one or two causes. Firstly a bad joint between the filler bowl (21) and the casing in which event tighten to correct matters (if this does not work a washer will have to be fitted) or secondly a loose petrol union on the suction pipe - i.e. any point between the bottom union of the pump and the petrol tank. Look at the washer between the the filter bowl and its bolt (20).
A sticking foot value is a very rare source of trouble. To rectify remove the filler bowl (21), filter (8) and foot valve (25) by means of a tommy bar through one of the holes. The foot valve can then be cleaned. A second filter (24) will be found in the foot valve underneath the priming tube (26).
If the pump works slowly without delivering petrol, this is due either to a blocked petrol pipe or filters (in which case the the filters or pipe must be cleaned out) or the battery is run down (in which case fill the the float chamber or the pump with petrol). This will probably enable the engine to be started by hand, and as the dynamo starts it will boost the battery sufficiently to run the pump.
If after reassembly the pump works, but does not deliver petrol, prime it by pouring a small amount of petrol into the top chamber. If no petrol is available, a few squirts of thin oil down the tube of the pump after removing the float (18) and the top plunger (11) will have the same effect.
Should the pump not work at all the trouble will be due to:
(1) A bad contact. To test this, remove the terminal from the pump and touch the wire to the pump body. If there is a bright flash this is in order. If not, then the battery could be run down due to bad connections somewhere in the system.
(2) The pump plungers (3 or 11) sticking due to dirt or grit getting between the plump plunger and the body. A blow on the pump with the fist may be enough to get it working and the dirt will pass through. If this doesn't work, remove the filter bowl (21) and foot valve (25): also the top of the pump and the cork float, and push the plunger (3) through the bottom, after which a clean rag can be drawn through the bore of the pump. When assembling, care should be taken to see that the plunger is fitted into the pump with the valve on top.
Electronically, the only thing that could cease to function is a broken wire or discharged battery. The connections can be seen in the illustration. To gain access to the electrical part of the pump remove the filter bowl (21), foot valve (25), unscrew the large hexagon nut (23) holding the inlet ring, draw off the casing and the internal parts of the electrical equipment can be inspected.
Care must be taken that the cork gland washer is in perfect condition.
When the casing is removed, take care not to break the wires and make sure that the top wire does not come across the rocking contact plate (13). A simple test to ensure the contacts are in working order is to remove the cap (22) from the top of the pump and lift the float (18) up and down its full stroke. Listen for the rocker plate click as it breaks its contact.
Filter. Clean the filter and filter bowl periodically to ensure free delivery of petrol. The filter is removed by withdrawing the attachment screw (20). Removing the bowl reveals the filter which can be cleaned with a stiff brush and petrol. Do not use a rag.