Spanner Conversion Chart - Find the socket or spanner to fit. MM, AF, BSW and BSF conversion chart.
Components
Ignition
Ignition Coil
Systems
Ignition
Points/Condenser - The traditional ignition system fitted to most cars before 1980.
Step-by-Step Guides
In this section, you will find all of the Classic Mechanic's tutorials to help you keep your car in good running order. The guides are split into three main sections to : Maintenance Guides cover the routine maintenance that should be carried out on a regular basis, Troubleshooting Guides provide you with vital information to help diagnose and cure problems, whilst the Overhaul Guides provide detailed information when individual components or systems need to be reconditioned.
Maintenance Guides
Ignition
HT Circuit - Guide to maintaining the spark plugs, rotor arm and HT leads.
Points/Condenser - Step-by Step guide to the routine maintenance of a traditional ignition system.
Troubleshooting Guides
Fueling
SU Carburettors - A guide to adjusting and setting SU carburettors, including balancing multi-carb setups.
Ignition
Compensating for Modern Fuels - Effectively set ignition timing to compensate for modern unleaded.
High-Tension Circuit - Troubleshooting the often overlooked high-tension circuit.
Ignition Coil - How to test an ignition coil.
Points/Condenser - How to test and troubleshoot the low-tension circuit in a traditional ignition system.
Spark Plugs - Using spark plugs as a diagnostic test to assess the engine's overall condition.
Overhaul Guides
Fueling
SU Carburettors - A guide to rebuilding and setting SU carburettors.
Ignition
Points/Condenser - How to rebuild and set up the low-tension side of a traditional ignition system.
Electronic Ignition Conversion - A guide to fitting electronic ignition (EI), including avoiding common pitfalls.
HT Circuit - Guide to replacing the HT circuit, including how to identify the best spark plug for your car.
Whilst every effort has been made to make these guides as helpful and informative as possible, The Classic Mechanic and its contributors accept no liability for loss, damage or injury caused by any errors in, or omissions from, the information given.
If you are not confident in undertaking work on your vehicle, seek professional advice from a qualified mechanic.
Maintenance Guides
Ignition
HT Circuit - Guide to maintaining the spark plugs, rotor arm and HT leads.
Points/Condenser - Step-by Step guide to the routine maintenance of a traditional ignition system.
Troubleshooting Guides
Fueling
SU Carburettors - A guide to adjusting and setting SU carburettors, including balancing multi-carb setups.
Ignition
Compensating for Modern Fuels - Effectively set ignition timing to compensate for modern unleaded.
High-Tension Circuit - Troubleshooting the often overlooked high-tension circuit.
Ignition Coil - How to test an ignition coil.
Points/Condenser - How to test and troubleshoot the low-tension circuit in a traditional ignition system.
Spark Plugs - Using spark plugs as a diagnostic test to assess the engine's overall condition.
Overhaul Guides
Fueling
SU Carburettors - A guide to rebuilding and setting SU carburettors.
Ignition
Points/Condenser - How to rebuild and set up the low-tension side of a traditional ignition system.
Electronic Ignition Conversion - A guide to fitting electronic ignition (EI), including avoiding common pitfalls.
HT Circuit - Guide to replacing the HT circuit, including how to identify the best spark plug for your car.
Whilst every effort has been made to make these guides as helpful and informative as possible, The Classic Mechanic and its contributors accept no liability for loss, damage or injury caused by any errors in, or omissions from, the information given.
If you are not confident in undertaking work on your vehicle, seek professional advice from a qualified mechanic.
Overhauling Points/Condenser Ignition Systems
Anybody who has ventured under the distributor cap of a classic car will be familiar with the traditional points/condenser ignition system. This was the standard setup on the vast majority of cars built before 1980, and at first glance appears to be an endlessly complicated array of switches and wires. However, in operating principle the traditional points/condenser ignition system is a very simple affair, and quite easy to maintain. Nevertheless, it also very delicate, requiring repeated and regular adjustment to maintain good running, performance and economy figures.
The following Tutorial is for Lucas distributors as fitted to the majority of British cars from the 1940s - 1980s, although the procedure for other makes is similar. If in doubt, consult your car's service manual.
Before carrying out any work on the ignition system, it is essential that the battery should be disconnected.
Tools
1 - Inspect
Before thinking about replacing your existing components, it is a good idea to take a look at the old ones before removing them. Careful inspection can reveal a lot about the condition of your ignition system, and help to diagnose any problems you may have been having.
Refer to the Troubleshooting article for a Points/Condenser ignition system in the Step-by-Step Guides section on how to do this.
2 - Remove Components
Remove the distributor cap and lift off the rotor arm. When removing the distributor components, it is a good idea to take them out as one complete assembly. Begin by removing the points securing/adjustment screw and set it aside. Next remove the small screw holding the capacitor to the distributor base plate. By sliding the low-tension block connector upwards, it should be possible to remove the whole assembly.
3 - Strip Distributor
If it has been an unknown duration since the distributor was last serviced, it is a good idea to strip it further to inspect for damage or wear. If this is not considered necessary, proceed to step 4.
Remove the two small screws securing the distributor base-plate to the body - these are located in the small recesses at the edges of the plate. With a pair of long-nose pliers, carefully lift the vacuum advance spring off the pin. Remove the whole distributor base plate as one unit.
Rotate the two halves of the base plate against each other until they separate. Inspect the braided electrical wire for wear, check that the sprung steel bracket is intact, and clean the whole assembly thoroughly with a degreasing agent.
Next inspect the inside of the distributor body. Check that there is no play in the distributor drive shaft - any sideways movement indicates excessive wear, and the whole unit should be removed for a complete overhaul. Check that the centrifugal weights operate correctly (they should splay outwards with a clean, crisp movement when the distributor drive shaft is turned in reverse), and that the springs are in good condition and secure. Ensure that there is no foreign matter in the base of the distributor - over a period of years, small grub screws and nuts may have been dropped down during a routine point/condenser change. Remove any loose matter, as this can cause terminal wear if left.
If any of the parts in the distributor base are worn or damaged excessively, the distributor must be removed from the car and overhauled separately.
As far as possible, the inside of the distributor body should be cleaned with a lint-free rag. DO NOT pour any degreasant or other cleaning agents into the body as they could easily seep into the engine oil and contaminate it.
Rub a little engine oil onto the centrifugal weights and springs, and pour no more than a teaspoonful into the base of the distributor to lubricate the drive shaft.
Reassemble the distributor base plate, clean the area around the securing screws and reinstall it. Ensure that the screws are tight, clean and have their washers, and that the base plate wire is attached with a good, clean connection (use emery paper if necessary to achieve a clean contact). Carefully reattach the vacuum advance spring.
4 - Test for Continuity
With the distributor base plate attached to the distributor body, use a multimeter and check for electrical continuity.
First check that there is no electrical resistance between the distributor base plate and the body. If there is any other reading besides nil, it is almost certainly the distributor base plate wire that is at fault. This should be replaced as a matter of course if there is any visible wear to its insulation or contacts.
Next check that there is no resistance between the distributor and the nearest earth point on the vehicle. Normally, there is one or more a large earth straps linking the engine to the chassis or engine bay. Wipe away any surface dirt and check for continuity between this and the distributor base plate.
If the multimeter is showing any resistance at this point, the distributor is incorrectly earthed and must be removed for inspection, cleaning and reinstalling.
Finally, check the wire between the distributor low-tension block and the negative side of the ignition coil for continuity.
5 - Install New Components
When changing any components inside the distributor, it is false economy not to replace the entire switching system as a matter of course.
Begin by taking a fresh set of contact breaker points. Undo the nut securing the sprung steel arm to the pin and lift away the plastic insulation spacer. Carefully prise the sprung steel arm off the pin and open the points to inspect the electrical contacts. These should be clean, free from grime, oil and dirt, and have no surface cracks. Any imperfections in the points at this stage is bound to lead to ignition trouble, and they should be discarded. Ensure that the second plastic insulation spacer remains at the bottom of the pin before reassembly.
Close the points and prise the sprung steel arm back over the pin. Take a new condenser and place the ring contact at the end of the wire over the pin on the points so that it is in direct contact with the steel of the points arm. Next take a new low-tension block and place the ring contact end of its wire over the same pin so that it too is in electrical contact with both the points and condenser.
Place the plastic insulating spacer over the top of the pin with the 'shoulder' side facing downwards. Carefully align the ring contacts of the condenser and low-tension block so that they slip over the shoulder of this plastic spacer. This is a critical stage of the reassembly process - the insulation spacer is designed to prevent the any of the components coming into contact with the points pin, which would result in the whole system shorting to the distributor base plate.
With each of the components' ring contacts correctly located, screw the securing nut onto the pin and tighten it by hand.
Install the points on the distributor base plate by screwing them loosely down with the securing screw. Slide the low-tension block into the slot at the front of the distributor body and screw down the condenser. It is vital to ensure that none of the wires come into contact with the cam at this point. Where possible, they should be routed around the outside of the distributor base plate.
When satisfied that the wiring is correctly located, tighten the securing nut on top of the points pin and screw the condenser down fully. Reattach the wire from the negative side of the coil to the low-tension block's spade connector.
6 - Set the Points
Setting the correct points gap is vital in ensuring the primary ignition circuit is broken cleanly and consistently, and remains closed for long enough to allow the primary coil to charge sufficiently to yield a strong spark.
Rotate the engine by hand until the highest point of one of the cam's corner lobes is in contact with the arm on the breaker points. Place a broad bladed screwdriver between the small cutout on the points mounting plate and the pin on the distributor base plate. With another screwdriver, carefully slacken the points securing screw and twist the other screwdriver until the points are open by between 14 and 16 thousandths of an inch (0.0014" - 0.0016"). A set of feeler gauges are essential for this job, but in their absence, a piece of cardboard from a cereal box will suffice in the short-term.
With the points gap, set tighten the securing screw and rotate the cam to ensure that the points are opened by an equal distance with each corner on the cam. Variation in the opening gaps by more than 2 thou (0.0002") will give rough running, particularly at higher engine speeds, and requires the distributor to be removed and rebuild.
7 - Test the System
With the new ignition system fitted, it is a wise idea to carry out a final set of checks before attempting to start the engine.
First ensure that all of the wiring is correctly located, that the coil is connected to both the positive and the low-tension block on the distributor body.
Next close the points by rotating the cam and use a multimeter to check for continuity between the negative side of the coil and the distributor base plate. There should be no resistance. Rotate the cam further to open the points fully and check with a multimeter again. There should be infinite resistance, as the connection should be broken.
Any divergence from these readings at this stage suggests that the ignition components have been incorrectly located, and they must be removed, inspected and reinstalled.
Rub a little engine oil onto the faces of the cam to lubricate it and minimize rapid wear of the points.
Inspect the rotor arm for signs of black deposits (this is indicative of a fault on the high-tension side of the ignition, and must be remedied separately to the points/condenser overhaul - see the overhaul guide to rebuilding the HT circuit for more information). With a clean lint-free rag, carefully wipe the brass top surface of the rotor arm to remove any build up of grime. If it is necessary to clean the edge of the rotor arm, gently rub the edge against a clean tyre wall in straight lines. Under no circumstances should any abrasive force be applied to the rotor arm - this would result in difficulty transferring an electrical pulse cleanly and cause sparking inside the distributor cap rather than the spark plugs.
Carefully wipe out the inside of the distributor cap and gently pull the carbon brush out by a fraction to ensure a good contact with the rotor arm. Seat the rotor arm securely on top of the drive shaft and replace the distributor cap, ensuring the HT leads are all correctly connected.
Reattach the battery and start the engine. Over a carefully planned route, test the engine's responsiveness under hard acceleration. Evidence of pinking or sluggish performance may be indicative of incorrectly set timing, which may have occurred when the distributor body was being handled during cleaning. Reset the timing and assess again.
8 - Maintenance
As an unfortunate side effect of their complexity, traditional points/condenser ignition systems are prone to becoming out of tune with use, and require regular maintenance to ensure peak performance.
Every fortnight or 250 miles, remove the distributor cap and rotor arm, and check the points gap. Over time the contact surfaces degrade and recede, increasing the gap. Reset to 15 thou and inspect the rest of the components for signs of black deposits. If these are present, consult the Troubleshooting section to remedy the problem.
For more information on routine maintenance of your points/condenser ignition system, see the Troubleshooting section in the Step-by-Step Guides section.
The following Tutorial is for Lucas distributors as fitted to the majority of British cars from the 1940s - 1980s, although the procedure for other makes is similar. If in doubt, consult your car's service manual.
Tools
- Assortment of screwdrivers
- Small socket/adjustable wrench
- Long-nose pliers
- Feeler gauge (0.0014" - 0.0016")
- Multimeter
- Emery paper
- Degreasing agent
- Lint-free rag
- Capful of engine oil.
1 - Inspect
Before thinking about replacing your existing components, it is a good idea to take a look at the old ones before removing them. Careful inspection can reveal a lot about the condition of your ignition system, and help to diagnose any problems you may have been having.
Refer to the Troubleshooting article for a Points/Condenser ignition system in the Step-by-Step Guides section on how to do this.
2 - Remove Components
Remove the distributor cap and lift off the rotor arm. When removing the distributor components, it is a good idea to take them out as one complete assembly. Begin by removing the points securing/adjustment screw and set it aside. Next remove the small screw holding the capacitor to the distributor base plate. By sliding the low-tension block connector upwards, it should be possible to remove the whole assembly.
3 - Strip Distributor
If it has been an unknown duration since the distributor was last serviced, it is a good idea to strip it further to inspect for damage or wear. If this is not considered necessary, proceed to step 4.
Remove the two small screws securing the distributor base-plate to the body - these are located in the small recesses at the edges of the plate. With a pair of long-nose pliers, carefully lift the vacuum advance spring off the pin. Remove the whole distributor base plate as one unit.
Rotate the two halves of the base plate against each other until they separate. Inspect the braided electrical wire for wear, check that the sprung steel bracket is intact, and clean the whole assembly thoroughly with a degreasing agent.
Next inspect the inside of the distributor body. Check that there is no play in the distributor drive shaft - any sideways movement indicates excessive wear, and the whole unit should be removed for a complete overhaul. Check that the centrifugal weights operate correctly (they should splay outwards with a clean, crisp movement when the distributor drive shaft is turned in reverse), and that the springs are in good condition and secure. Ensure that there is no foreign matter in the base of the distributor - over a period of years, small grub screws and nuts may have been dropped down during a routine point/condenser change. Remove any loose matter, as this can cause terminal wear if left.
If any of the parts in the distributor base are worn or damaged excessively, the distributor must be removed from the car and overhauled separately.
As far as possible, the inside of the distributor body should be cleaned with a lint-free rag. DO NOT pour any degreasant or other cleaning agents into the body as they could easily seep into the engine oil and contaminate it.
Rub a little engine oil onto the centrifugal weights and springs, and pour no more than a teaspoonful into the base of the distributor to lubricate the drive shaft.
Reassemble the distributor base plate, clean the area around the securing screws and reinstall it. Ensure that the screws are tight, clean and have their washers, and that the base plate wire is attached with a good, clean connection (use emery paper if necessary to achieve a clean contact). Carefully reattach the vacuum advance spring.
4 - Test for Continuity
With the distributor base plate attached to the distributor body, use a multimeter and check for electrical continuity.
First check that there is no electrical resistance between the distributor base plate and the body. If there is any other reading besides nil, it is almost certainly the distributor base plate wire that is at fault. This should be replaced as a matter of course if there is any visible wear to its insulation or contacts.
Next check that there is no resistance between the distributor and the nearest earth point on the vehicle. Normally, there is one or more a large earth straps linking the engine to the chassis or engine bay. Wipe away any surface dirt and check for continuity between this and the distributor base plate.
If the multimeter is showing any resistance at this point, the distributor is incorrectly earthed and must be removed for inspection, cleaning and reinstalling.
Finally, check the wire between the distributor low-tension block and the negative side of the ignition coil for continuity.
5 - Install New Components
When changing any components inside the distributor, it is false economy not to replace the entire switching system as a matter of course.
Begin by taking a fresh set of contact breaker points. Undo the nut securing the sprung steel arm to the pin and lift away the plastic insulation spacer. Carefully prise the sprung steel arm off the pin and open the points to inspect the electrical contacts. These should be clean, free from grime, oil and dirt, and have no surface cracks. Any imperfections in the points at this stage is bound to lead to ignition trouble, and they should be discarded. Ensure that the second plastic insulation spacer remains at the bottom of the pin before reassembly.
Close the points and prise the sprung steel arm back over the pin. Take a new condenser and place the ring contact at the end of the wire over the pin on the points so that it is in direct contact with the steel of the points arm. Next take a new low-tension block and place the ring contact end of its wire over the same pin so that it too is in electrical contact with both the points and condenser.
Place the plastic insulating spacer over the top of the pin with the 'shoulder' side facing downwards. Carefully align the ring contacts of the condenser and low-tension block so that they slip over the shoulder of this plastic spacer. This is a critical stage of the reassembly process - the insulation spacer is designed to prevent the any of the components coming into contact with the points pin, which would result in the whole system shorting to the distributor base plate.
With each of the components' ring contacts correctly located, screw the securing nut onto the pin and tighten it by hand.
Install the points on the distributor base plate by screwing them loosely down with the securing screw. Slide the low-tension block into the slot at the front of the distributor body and screw down the condenser. It is vital to ensure that none of the wires come into contact with the cam at this point. Where possible, they should be routed around the outside of the distributor base plate.
When satisfied that the wiring is correctly located, tighten the securing nut on top of the points pin and screw the condenser down fully. Reattach the wire from the negative side of the coil to the low-tension block's spade connector.
6 - Set the Points
Setting the correct points gap is vital in ensuring the primary ignition circuit is broken cleanly and consistently, and remains closed for long enough to allow the primary coil to charge sufficiently to yield a strong spark.
Rotate the engine by hand until the highest point of one of the cam's corner lobes is in contact with the arm on the breaker points. Place a broad bladed screwdriver between the small cutout on the points mounting plate and the pin on the distributor base plate. With another screwdriver, carefully slacken the points securing screw and twist the other screwdriver until the points are open by between 14 and 16 thousandths of an inch (0.0014" - 0.0016"). A set of feeler gauges are essential for this job, but in their absence, a piece of cardboard from a cereal box will suffice in the short-term.
With the points gap, set tighten the securing screw and rotate the cam to ensure that the points are opened by an equal distance with each corner on the cam. Variation in the opening gaps by more than 2 thou (0.0002") will give rough running, particularly at higher engine speeds, and requires the distributor to be removed and rebuild.
7 - Test the System
With the new ignition system fitted, it is a wise idea to carry out a final set of checks before attempting to start the engine.
First ensure that all of the wiring is correctly located, that the coil is connected to both the positive and the low-tension block on the distributor body.
Next close the points by rotating the cam and use a multimeter to check for continuity between the negative side of the coil and the distributor base plate. There should be no resistance. Rotate the cam further to open the points fully and check with a multimeter again. There should be infinite resistance, as the connection should be broken.
Any divergence from these readings at this stage suggests that the ignition components have been incorrectly located, and they must be removed, inspected and reinstalled.
Rub a little engine oil onto the faces of the cam to lubricate it and minimize rapid wear of the points.
Inspect the rotor arm for signs of black deposits (this is indicative of a fault on the high-tension side of the ignition, and must be remedied separately to the points/condenser overhaul - see the overhaul guide to rebuilding the HT circuit for more information). With a clean lint-free rag, carefully wipe the brass top surface of the rotor arm to remove any build up of grime. If it is necessary to clean the edge of the rotor arm, gently rub the edge against a clean tyre wall in straight lines. Under no circumstances should any abrasive force be applied to the rotor arm - this would result in difficulty transferring an electrical pulse cleanly and cause sparking inside the distributor cap rather than the spark plugs.
Carefully wipe out the inside of the distributor cap and gently pull the carbon brush out by a fraction to ensure a good contact with the rotor arm. Seat the rotor arm securely on top of the drive shaft and replace the distributor cap, ensuring the HT leads are all correctly connected.
Reattach the battery and start the engine. Over a carefully planned route, test the engine's responsiveness under hard acceleration. Evidence of pinking or sluggish performance may be indicative of incorrectly set timing, which may have occurred when the distributor body was being handled during cleaning. Reset the timing and assess again.
8 - Maintenance
As an unfortunate side effect of their complexity, traditional points/condenser ignition systems are prone to becoming out of tune with use, and require regular maintenance to ensure peak performance.
Every fortnight or 250 miles, remove the distributor cap and rotor arm, and check the points gap. Over time the contact surfaces degrade and recede, increasing the gap. Reset to 15 thou and inspect the rest of the components for signs of black deposits. If these are present, consult the Troubleshooting section to remedy the problem.
For more information on routine maintenance of your points/condenser ignition system, see the Troubleshooting section in the Step-by-Step Guides section.
Points/Condenser Ignition Explained
The ignition is one of the four vital systems in your engine's operation. Along with fueling, cooling and lubrication, it would simply not be possible to run your engine without it. Its purpose is to ignite the air/fuel mixture in the engine's cylinders and begin the power stroke.
Almost every car built in Britain before 1980 used the traditional Points/Condenser ignition system shown below. Although prone to unreliability due to the number of components needed, in operating principle it is a relatively simple affair.
The ignition system is comprised of two separate circuits:
1. The low-tension (primary) circuit, which generates the spark.
2. The high-tension (secondary) circuit, which delivers the spark to the relevant spark plug.
The Cycle
As the distributor drive shaft continues to rotate, and the corner lobe of the cam moves away, the contact breaker points will spring shut, completing the primary circuit and beginning the process of charging the primary coil once again.
As the rotor arm is connected to the same drive shaft as the cam, the two rotate at the same rate. Therefore, at the point that the next corner lobe of the cam is opening the contact breaker points, the rotor arm will be aligned with the next contact ready to distribute the impending pulse to the next spark plug in the engine's cycle.
The cycle continues in this manner of charging and discharging the coils indefinitely. In a 4-stroke, 4-cylinder engine (as shown in the diagram), for every 1000 revolutions of the engine, the ignition cycle will be completed 2000 times.
For more information on the individual components in a points/condenser ignition system, see the separate articles in the Components Explained section.
To see how to dismantle, test and rebuild a points/condenser ignition system, check out the Tutorial article in the Step-by-Step Guides section.
Almost every car built in Britain before 1980 used the traditional Points/Condenser ignition system shown below. Although prone to unreliability due to the number of components needed, in operating principle it is a relatively simple affair.
The ignition system is comprised of two separate circuits:
1. The low-tension (primary) circuit, which generates the spark.
2. The high-tension (secondary) circuit, which delivers the spark to the relevant spark plug.
The Ignition Coil
The heart of the ignition system is the ignition coil. It is in this sealed unit that the high-voltage electrical pulse necessary to create an ignition spark is generated. The ignition coil contains two separate coils of wire arranged concentrically (one inside the other). The outer, primary coil (1) is made up of around 200 turns of thick wire, whilst the inner, secondary coil (2) comprises of around 20,000 turns of much thinner wire. Using the transformer effect, charge can be transferred from the primary to the secondary coil, converting 12v of electricity from the car's battery into the 10,000v that creates the spark.
The Primary Circuit
The Primary Circuit
With the contact breaker points closed, a 12v current flowing from the car's battery induces an electro-magnetic charge in the primary coil (1). At the same time, a small capacitor, or 'condenser', placed in parallel to the circuit is also charged for an auxiliary purpose.
As the engine-driven distributor drive shaft rotates, a corner lobe of its cam pushes the contact breaker points open, breaking the primary circuit and preventing the primary coil from charging any further. The condenser acts as dampener at this point, preventing current from arcing (jumping) across the open gap in the breaker points, ensuring that the circuit is broken cleanly.
The Secondary Circuit
Once the primary circuit is broken, the electro-magnetic field in the primary coil (1) becomes unstable and begins to collapse. This charge is then transferred to the inner, secondary coil (2), at which point the transformer effect drastically increases the voltage from 12v to around 10,000v, whilst proportionally decreasing the amperage. This pulse of high-voltage electricity is then taken away from the secondary coil through a high-tension lead to the rotor arm, which is mounted on top of the rotating distributor drive shaft. At this point, the rotor arm is aligned with one of its contacts. The pulse passes through this contact and along a second high-tension lead to the spark plug. At the end of the spark plug is a small gap, the other side of which is earthed to the negative side of the battery through the vehicle's engine block. As the pulse jumps across this gap, it effectively short-circuits and a large spark results, igniting the air/fuel mixture in the cylinder and beginning the engine's power stroke.
As the engine-driven distributor drive shaft rotates, a corner lobe of its cam pushes the contact breaker points open, breaking the primary circuit and preventing the primary coil from charging any further. The condenser acts as dampener at this point, preventing current from arcing (jumping) across the open gap in the breaker points, ensuring that the circuit is broken cleanly.
The Secondary Circuit
Once the primary circuit is broken, the electro-magnetic field in the primary coil (1) becomes unstable and begins to collapse. This charge is then transferred to the inner, secondary coil (2), at which point the transformer effect drastically increases the voltage from 12v to around 10,000v, whilst proportionally decreasing the amperage. This pulse of high-voltage electricity is then taken away from the secondary coil through a high-tension lead to the rotor arm, which is mounted on top of the rotating distributor drive shaft. At this point, the rotor arm is aligned with one of its contacts. The pulse passes through this contact and along a second high-tension lead to the spark plug. At the end of the spark plug is a small gap, the other side of which is earthed to the negative side of the battery through the vehicle's engine block. As the pulse jumps across this gap, it effectively short-circuits and a large spark results, igniting the air/fuel mixture in the cylinder and beginning the engine's power stroke.
The Cycle
As the distributor drive shaft continues to rotate, and the corner lobe of the cam moves away, the contact breaker points will spring shut, completing the primary circuit and beginning the process of charging the primary coil once again.
As the rotor arm is connected to the same drive shaft as the cam, the two rotate at the same rate. Therefore, at the point that the next corner lobe of the cam is opening the contact breaker points, the rotor arm will be aligned with the next contact ready to distribute the impending pulse to the next spark plug in the engine's cycle.
The cycle continues in this manner of charging and discharging the coils indefinitely. In a 4-stroke, 4-cylinder engine (as shown in the diagram), for every 1000 revolutions of the engine, the ignition cycle will be completed 2000 times.
For more information on the individual components in a points/condenser ignition system, see the separate articles in the Components Explained section.
SU Carburettors
The SU variable jet carburettor is a marvel of engineering simplicity. However, despite this, it is a very delicate component with fine tolerances that must be kept in top condition adjusted precisely in order to function correctly. A properly adjusted variable jet carburettor delivers economy, power and
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