The ignition system consists of two circuits-primary and secondary. The primary circuit includes the battery, ignition switch, the primary or low-tension circuit of the coil and the distributor contact breaker and capacitor. The secondary circuit includes the secondary of high-tension circuit of the coil, the distributor rotor and cover segments, the high-tension cables and the sparking plugs.
The ignition coil, which is mounted on the right-hand side of the engine, consists of a soft iron core around which is wound the primary and secondary windings. The coil carries at one end a centre high-tension terminal and two 10W tension terminals marked (SW) (switch) and (CB) (contact breaker) respectively.
The ends of the primary winding are connected to the (SW) and (CB) terminals and the secondary Winding to the (CB) terminal and the high-tension terminal.
The distributor is mounted on the right-hand side of the engine and is driven by a shaft and helical gear from the camshaft. Automatic timing control of the distributor is controlled by a centrifugal mechanism and a vacuum operated unit each operating entirely independently of each other. The centrifugal mechanism regulates the ignition advance according to engine speed, while the vacuum control varies the timing according to engine load. The combined effect of the two mechanisms gives added efficiency over the full operating range of the engine. A micrometer adjuster is provided to give a timing adjustment to allow for the engine condition and the grade of used.
A keyed moulded rotor with a metal electrode is mounted on top of the cam. Attached to the distributor body above the centrifugal advance mechanism is a contact breaker plate carrying the contact breaker points and a capacitor connected in parallel. A cover is fitted over the distributor body and retained by two spring clips attached to the body.
Inside the cover is a centre electrode and spring- loaded carbon brush which makes contact with the rotor. The brush is of composite construction, the top portion being made of a. resistive compound, the lower portion is made of softer carbon to prevent wear of the rotor electrode. Under no circumstances must a short non-resistive brush be used to replace this long resistive type. A measure of radio interference suppression is given by this brush.
Spaced circumferentially around the centre electrode are the sparking plug cable segments. The distributor is secured in position on the cylinder block by a clamp plate.
The sparking plugs are located on the right-hand side of the engine and have a 14 mm. thread with a 3/4 in. reach.
When the ignition is switched on, the current from the battery :Hows through the primary circuit, and a magnetic field is built up around the core of the coil. When the contact breaker points are opened by rotation of the distributor cam, the current flow is interrupted, causing a high voltage to be induced in the secondary winding of the coil by sudden collapse and consequent change in the magnetic field. The hightension current thus generated in the secondary winding of the coil is conveyed by the coil high-tension cable to the centre terminal of the distributor cover. From here the current passes through the carbon brush to the rotor, where the high-tension current passes along the rotor electrode and is distributed to the segments and thence to the sparking plugs Via the high-tension cables.
The purpose of the following adjustments is to maintain efficient engine performance and economical running.
Gap adjustment should be made by bending the side electrode only. Never bend the central electrode. If the plugs are dirty, damaged or excessively burned, see Section B.1.
Adjust the contact breaker points at the recommended intervals as follows:- Remove the distributor cover and rotor. Rotate the engine with the starting handle until the fibre heel of the rocker is on the peak of one of the cam lobes. The gap of the contact breaker points should be within .014 to .016 in. (.36 to .40 mm.). Gap adjustment should be made by slackening the fixed contact plate securing screws and moving the plate the gap gauge is a sliding between the two contacts. Tighten the securing screws and recheck the gap. Replace the rotor and cover. if the points are dirty or pitted see Section B.5.
Adjust the ignition timing, if the distributor has been disturbed, as follows: - Remove the valve rocker cover so that the valve action can be observed. Rotate the engine with the starting handle until No. 1 piston is at the top of its compression stroke (i.e. the exhaust valve of No. 6 cylinder is just closing and the inlet valve just opening). Turn the crankshaft until the recess in the crankshaft pulley flange is in line with the pointer on the timing chain cover (See Fig. B.2). If the timing chain cover has been removed, align the bright links on the timing chain with the marked teeth on the camshaft and crankshaft sprockets (see Section A.25) when No. 6 and No. 1 pistons will be at T.D.C. Set the micrometer adjustment on the distributor to its central position. The crankshaft should now be rotated backwards to obtain its correct position before setting the distributor points, this setting is correct for premium grade fuels only. With the cover removed the distributor body must be rotated until the rotor arm is pointing to the position of No. 1 electrode in the cover. With the contact points just opening, tighten the clamp plate bolt.
Finer adjustment can be obtained under road conditions, by means of the micrometer adjustment. Note this adjustment should not be used for initial setting of the ignition; it is only altered if the main setting requires adjustment to meet the characteristics of the grades of petrol being used. There is a considerable amount of latitude for adjustment, but only extremely small movement of the adjustment knob should be made at one time.
Replace the distributor cover and cylinder head cover.
If the ignition System fails, or misfiring occurs, first make sure that the trouble is not due to defects in the engine, carburetter 01’ fuel supply. Faults should be diagnosed by applying the following tests -
Examine the cables, the cables from the coil to the distributor, and from the distributor to the plugs. If the rubber insulation shows signs of deterioration or cracking, the cable should be renewed.
Test the plugs and high-tension cables by removing the plugs in turn and allowing them to rest on the cylinder head or other convenient earthing point, and observing whether a spark occurs at the points when the engine is turned by hand. It should, however, be noted that this is only a rough test, since it is possible that a spark may not take place when the plug is under compression. If necessary, clean and test the plugs, using at plug cleaning and testing machine.
To trace a fault in the circuit, release the instrument panel from the dash, switch on the ignition, and turn the engine until the distributor contacts are opened. Refer to the wiring diagram, Fig. N.25, and, with the aid of a voltmeter (0 to 20), check the circuit as follows -
Cable (brown) - Starter switch to fuse unit A.1 terminal
Connect the voltmeter between the fuse unit A.1 terminal and earth. No reading indicates a faulty cable or loose connection.
Connect the voltmeter between the control box terminal (A.1) and earth. No reading indicates a faulty control box.
Cable (brown and blue) - Control box to lighting and ignition switch
Connect the voltmeter between the lighting switch terminal (A) and earth. No reading indicates a faulty cable or loose connection.
Connect the voltmeter between the ignition switch (white cable terminal) and earth. No reading indicates a faulty ignition switch.
Cable (white) - Ignition switch to fuse unit A.3 terminal
Connect the voltmeter between the fuse unit A.3 terminal and earth. No reading indicates a faulty cable or loose connection.
Cable (white) - Fuse unit A.3 terminal to ignition coil
Connect the voltmeter between the ignition coil terminal (SW) and earth. No reading indicates a faulty cable or loose connection.
Connect the voltmeter between the ignition coil terminal (CB) and earth. No reading indicates a faulty ignition coil.
Cable (white and black) - Ignition coil to distributor
Connect the voltmeter between the distributor terminal and earth. No reading indicates a faulty cable or loose connection.
Connect the voltmeter across the distributor contacts. If no reading is given, remove the capacitor and test again. if a reading is given, the capacitor is faulty.
The installation of the ignition coil is a reversal of the procedure
tracking, a conducting path may have formed between adjacent segments. This is indicated by a thin ‘black line between the segments; when this has occurred the cover should be renewed.
Reassembly of the distributor is 5. reversal of the procedure
To dismantle, noting the following points:-
Replacing the distributor to the reverse of the procedure
To Remove, noting the following points -
Replace the tachometer housing, rotating the external drive dog until it mates up with the slot in the distributor driving spindle. Ascertain that the smaller segment of the offset dog, situated within the tachometer housing is in the downward position.
Note: The internal dog should now be in the
twenty-to-two position, see Fig. B.5. Secure the tachometer housing to the block with its three setpins.
Replace the distributor following the instructions given in Section B.5.
The sparking plug gap (for type of plug see
General Data) should be maintained at .024 to .026 in. (.6096 to .660 mm). If the gap is allowed to become too wide, misfiring at high speeds is liable to occur; and if too small, bad slow running and idling will be the result.
Sparking plugs should be regularly inspected, cleaned and tested. This is of vital importance to ensure good engine performance, coupled with fuel economy.
When removing the plugs from the engine, use a box spanner, this will avoid possible damage to the insulators. Always remove the copper washers. The plugs should then be placed in a suitable holder which has holes drilled to admit the upper end of the plugs and marked to identify each one with the cylinder from which it was removed.
The plugs should now be carefully examined.
Oil fouling will be indicated by a Wet shiny black deposit on the insulator. This condition is usually caused by worn cylinders, pistons or gummed rings. Oil vapour is forced from the crankcase, during the suction stroke of the piston which fouls the plugs.
Petrol fouling will cause a dry, fluffy, black deposit to be apparent on the plugs. This is usually caused by faulty carburation, but a faulty coil or leaking and worn out ignition leads, may have the same
Under the above conditions, if the plugs otherwise appear to be sound, they should be cleaned thoroughly, adjusted, and tested.
When preparing for cleaning, the plug washers should be removed and examined. The condition of these washers is important in that a large proportion of the heat from the plug insulator is dissipated to the cylinder head by them. The washer should therefore be reasonably compressed. A loose plug can be easily overheated, thus shortening plug life. On the other hand, do not over-tighten. All that is needed is a good seal between the cylinder head and the plug. Tightening too much will cause distortion of the washer with the possibility of blow-by which will again lead to over- heating and resulting danger. If there is any question of defect, replace with new washers.
The plugs should now be thoroughly cleaned of all carbon deposit, resorting to scraping if necessary, removing as much as possible from the space between the insulator and shell. An oily plug should be washed out with petrol. If a plug cleaning machine is available, 5 to 10 seconds in this will remove all remaining signs of carbon. Remember to thoroughly blow-out the
plug after treatment under these conditions, in order to remove all traces of abrasive.
After cleaning, thoroughly examine the plug for cracked insulator or worn away insulator nose. Should either of these conditions be apparent a new plug should be installed.
Carbon deposit on the threads of the plugs, should be carefully removed by using a wire brush, or if available a wire buffing wheel. Take care not to damage the electrodes or insulator tip. Neglect of this cleaning operation will lead to tight threads and resultant loss of heat dissipation due to the carbon deposit, thereby causing overheating.
The condition of the electrodes should now be noted and any signs of corrosion removed, if it is felt that the plugs are worthy of further use. This can be carried out with the use of a small file to carefully dress the gap area. The gap should then be reset, to a Clearance of .024 to .026 in. (.6096 to .660 mm.). When resetting bend the side electrode only.
It is advisable whilst the plugs are under pressure in the testing machine, to apply a spot of oil to the terminal end, to check for air leakage. Excessive leakage here will tend to cause compression loss, rapid deterioration of the electrode and overheating of the electrode tip. The top half of the insulator should be carefully examined for any paint splashes or accumulation of grime and dust, which should be removed. Should there be any signs of cracks due to faulty use of the spanner, the plug should be renewed. When replacing the plug lead, make sure that it is securely attached.
It is recommended that sparking plugs should be cleaned and tested every 6,000 miles (9600 km.), and renewed at 12,000 miles (19200 km.).
Remember, plugs in good condition will ensure better fuel consumption and good engine performance.
Commencing at engine No. 6375, Champion N.5 plugs were fitted instead of Champion N.3 plugs. The N.3 plugs must however, be used for competition and high speed work.
|(a) Engine Will Not Fire||1||Battery discharged|
|2||Distributor contact points dirty, pitted or out of adjustment|
|4||Distributor carbon brush not in contact with cover|
|5||Loose connection in low-tension circuit|
|6||Distributor rotor arm cracked|
|(b) Engine Misfires||1||Distributor contact points dirty, pitted or our of adjustment|
|2||Contact breaker spring weak|
|5||Loose connection in low-tension circuit|
|6||High-tension cables cracked or perished|
|7||Sparking plug loose|
|8||Sparking plug insulation cracked|
|9||Sparking plug gap incorrect|
|10||Ignition timing too far advanced|