That compressed fuel/air mixture in the cylinder needs to be ignited at the right time in order initiate the biggest explosion possible!
The basic ignition system is comprised of coil, points, distributor and plugs.

Lets work backwards!!
The spark at the plug triggers the explosion hence the power stroke of the engine.
The spark represents a voltage spike of around 40,000volts which is supplied by the coil.
The high voltage spark is caused by a low voltage spike (12v) to the input of the coil.
The low voltage spike is caused by the rapid charge of a capacitor
The capacitor charge is initiated by the opening of the points.
The points are opened by a cam which is geared to the crankshaft position.

The above is repeated for each cylinder in turn and the function of the distributor (with its rotor arm)
is to connect the coil output to the appropriate spark plug. Most of our 'classic' engines have
four cylinders and the common firing order is 1,3,4,2

I think it can be stated without hesitation that the vast majority of difficulties with the engines
of our historic vehicles are due to ignition problems. It is amazing how often owners found
themselves struggling to fix elusive fuel problems for hours when in fact the problem
was 'dirty points', worn rotor arm, bad connection to the coil or loose HT cable! (I speak from personal

Tip 1

When fault-finding you can bypass the rotor arm and distributor cap by simply connecting a
spark plug directly to the output of the coil. No spark means the problem is the coil, points or
a connecting wire. The presence of a spark means the problem is distributor cap, rotor arm
or carbon pencil which feeds the rotor.

Tip 2

Remember that the chances are that a stationary engine is left with the points closed. If the
ignition is switched on then an electrical current will flow through the coil, through the points
and back to the battery. The coil may overheat and be damaged.
So... if the ignition needs to be switched on while some other electrical problem is diagnosed
then a good tip is to protect the coil by temporarily removing one of its low voltage spade


Below was provided by Phil Smith. It probably highlights a puzzle we all scratch our head over! I will carry out some research on the subject and add my findings later. If anyone can shed some light on the topic then please send it my way! Cheers, Tony tony.jones1948@outlook.com

Hi Tony,

You may like to consider the following for the tech. Section of the club website.

I have heard (or read) that wire cored HT leads should never be used with electronic ignition systems.

If this is true:

  • Is there a noticeable loss of performance with wire cored leads?
  • Is there a noticeable gain in performance with ribbon leads?
  • Will the use of wire cored leads cause early failure of the electronic system?
  • Is there a valid technical reason for this?
  • Is it marketing bulldust?

Cheers, Phil.

Feedback from Tony.

Whenever there is a spark electrical radio interference is transmitted. Typically this is percieved as noise on a radio, picture interference on a TV screen, failure of electronic ignition or malfunction of a computer.
Even more importantly, it can influence the behaviour of a modern engines ECU (Engine Control Unit)!
The process of reducing the level of electrical interference from a spark is known as 'suppression'.
The traditional (and simplest) 'suppressor' is merely a resister with a value of around 3,000 to 10,000 ohms.
If the resistance value is significantly less than 3,000ohms then it is unlikely to have a good supression of electrical noise.
If the resistance value is significantly higher than 10,000ohms then the intensity of the spark in the cylinder may be inadequate resulting in misfiring or poor performance.
The resistor suppressor can be located in the HT lead from the coil to the distributor or it may be located in the plug cap for every cylinder or it may located in the HT leads themselves or it may be located in each individual spark plug!
It is extremely important to appreciate that if an ignition system has more than one suppressor feeding a spark plug then the resistances are additive. eg If a suppressor is fitted in the plug caps and another is fitted in each spark plug then, assuming each has a resistance value of 10,000ohms then the total resistance is 20,000ohms per cylinder.
This value is not ideal and the consequence may be mysterious misfiring at high revs or general poor performance.
Never think that you are 'doing your car a favour' by adding multiple suppressors! You may instead be reducing its performance!

HT wire:
When our classics were manufactured all HT leads were simply copper wires with a good thick insulation. Such wire is easy to fit caps to etc and maintenance is a doddle.
As technology improved, HT wire became available which used various carbon mixes instead of copper. These mixes provided an electrical resistance which had the effect of reducing the electrical interference emitted ie behaved as an inbuilt suppressor.
This had the benefit of avoiding the need for any separate suppressor however, such wire can be difficult to add caps and connectors to and it is common for the connectors/caps to be moulded to the wire at time of manufacture. It is not unusual for some modern resistance wire to have values in excess of 25,000ohms. This may be ok for modern engines loaded with electronics but it is not what was planned by our classic car companies!

Plug Caps:
The suppression resistor may be built into the cap and markings on the cap would normally highlight the value in text.

Separate In-Line Suppressors:
These can appear it all sorts of shapes and sizes and would be identified in text.

Spark Plugs:
Manufacturers make spark plugs available with resistors build inside them. The coding of the plug would normally highlight whether or not a suppression resistor is inbuilt within the spark plug.
For example, any NGK plug which includes the letter 'R' in its code includes a resistor within it. BPR5ES plug is in fact a BP5ES with the addition of a resistor.

However you decide to suppress your ignition system and wherever you decide to locate the suppressor, it must be understood that, over time, the value of a resistor will probably increase so this is yet another area which we should monitor from time to time!

After reading through the above minefield we can see how easy it is to end up with inadequate suppression or mysterious misfiring/performance issues!

Good News!
It is quite easy to check the value of resistance by using a simple resistance meter. If you measure the resistance between the connection at the top of a spark plug and the electrode tip then if a resistance is incorporated into the plug, its value can be read.
A new resistance type of plug will have a value of around 3,000 - 5,000ohms. If an old plug reads significantly higher then it probably belongs in the bin.
The cap for the spark plug can also be checked using a meter. Similarly, the cables can be checked for resistance.

Having learned the above I decided to examine the ignition of my Rover 80. The wires are all simple copper. That's a good start because that's what Rover would have used originally.
Every spark plug cap was equipped with its own resistance suppressor.
Measurement of each suppressor cap provided readings between 15,000ohms and 25,000ohms. Not good! These values have dramatically increased (possibly over 60 years looking at the state of them!) from the original value. I think I may have just found the cause of mysterious misfiring events!
I have discarded the old suppressor caps and inserted a single 5,000ohm suppressor in the coil HT lead.
I now have an ignition system as near to original as possible while retaining adequate noise suppression.

The bottom line is:
Our classics were designed to expect copper HT wire so it is probably wise to avoid using modern resistive HT wire.
If your spark plugs incorporate suppression resistors within them then it is wise to avoid using any additional suppressors in the ignition system.
If your spark plugs do not incorporate resistors then a good move would be to either use plug caps which include suppressors OR install a single suppressor in the coild HT line.