Honda C90 - ignition system
Honda C90 - ignition system

The job of the ignition system is to produce the spark that ignites the fuel/air mixture in the engine combustion chamber.  It consists of a power source (either battery or a generator), a switching device to start and stop current flow at set intervals (contact breaker points or an electronic switch), a device to produce a high voltage (ignition coil) and a spark plug.   This is the wiring diagram for the ignition system of the 1978 C90 z2 super cub:

This is one of the electrical subsystems used on the C90s in the 60s and 70s. You can see the full wiring diagram and read about the basic principles of electricity in this post on the C90 electrical system

The spark is produced when an electric current jumps the gap between the spark plug electrodes.  

If you've ever received an electrostatic shock after walking on a nylon carpet then you will be familiar with the phenomenon:  the spark happens because because you have become electrically 'charged'[1] and the large potential difference (aka voltage) between you and the object you are reaching towards causes the molecules in the air – which  are normally poor conductors – to break apart into charged particles so that the gas becomes conductive (this process is called ionisation).   At this point the charge can “arc” across the gas.  The resulting electric current heats the gas and causes it to expand rapidly resulting in noise and light (a spark).

Honda used several types of ignition system in their super cubs - the version used in the 6v C90s (1965 to 1982) is battery ignition.  

Battery Ignition

Creating a spark requires potential energy of thousands of volts.  Since motorcycle electrical systems run at 6 or 12 volts a device is needed to transform the low voltage to a high voltage, and this is the job of the ignition coil. In the system used on the 6v C90s low voltage current flows from the battery to the ignition coil at intervals determined by the contact breaker points and is transformed by the ignition coil into a high voltage current which jumps the spark plug gap.

This type of system is called battery ignition or Kettering ignition (after the man who invented it in 1911,  Charles Kettering).  Until the mid 1960s introduction of the CM90, Honda super cubs used an even older system called magneto ignition that dated from the turn of the 20th Century.  Although there are pros and cons to each solution, when motorcycle manufacturers moved over to battery ignition in the 1950s and 1960s it was generally regarded as a more reliable (and cheaper) system.  

The main disadvantage of battery ignition is that if the battery is not sufficiently charged the quality of the spark is effected and this can cause the bike to run poorly.   Sometimes the resulting problems announce themselves when the battery is running low but still has enough charge to run the ignition.  When this happens the bike will run properly until an extra demand is placed on the battery - for instance when using the turn signal indicators - at which point the ignition system no longer has sufficient current and the engine stalls or misfires.

Honda used three other ignition systems on their super cubs (Magneto, Energy Transfer Ignition and CDI) and these are described below.  If you want to know about the parts used in the 6v battery ignition system you can read about them here.

Magneto ignition

This type of ignition was used on the early production run of the first super cub, the c100.

These bikes had an alternating current (AC) generator[2] with two coils, one for ignition and one for lighting.  

the early C100 (1958) ignition system. Note that tension is an old term for voltage. 

The rotating flywheel magnets generate an alternating current in the high tension coil and a high voltage is transmitted to the spark plug at regular intervals determined by the contact breaker (see the Ignition Coil section below for information on how the very high voltage needed to create a spark is induced).

The "light coil" is a low voltage coil that powers the head, tail & meter lamps and charges the battery.   Although not shown in the picture above, a battery was still fitted on the early c100s to run "safety features" like the horn, turn signal indicators and brake light.

The later run of the C100, and most of the other super cubs produced in the 60s,  used a type of ignition that Honda calls Energy Transfer.

Energy Transfer Ignition

This system uses a low voltage magneto and a separate AC coil to induce high voltage for the spark plug.  Honda call this system Energy Transfer (if you are interested in how it works you can read the explanation on p31 of Honda's Electrical System Manual).

Although both the battery and magneto systems were old designs, even in the late 50s/early 1960s when Honda started using them, nothing much better came along until the 1970s when the development of transistorised electrical components meant it was possible to replace the mechanical contact breaker points with an electronic switch.   Honda adopted the technology in the form of the Capacitor Discharge Ignition (CDI) system used on the 1983 12v C90C model and, a couple of years earlier, on the C70.

Capacitor Discharge Ignition

In this system an AC generator charges a capacitor[3] and a timer (pulse generator) tells the electronic switch in the CDI unit to connect the capacitor to the ignition coil.  As the capacitor discharges into the primary winding a high voltage is induced in the the ignition coil, creating the spark.

CDI ignition wiring diagram

This design resolves several maintenance issues with the battery ignition system used on the older 6v C90s as it eliminates the contract breaker points and the mechanical spark advance unit and, unlike the 6v engine, does not require a fully charged battery to run[4].

The components that make the 6v battery ignition work (see the wiring diagram above) are explained below:

Ignition Coil

the original C90-Z ignition coils were made by Tec (model number FL801-6V85)

As the electrical current passes from the battery through the ignition coil (which is made up of an iron core, primary and secondary coils) a magnetic field is created around the primary coil.

With thanks to Denso for the diagram

The magnetic field is effectively a store of energy which can then be converted back into electricity.   When a coil of wire is exposed to a magnetic field and the magnetic field then changes it creates an electric current in the coil of wire. This process is known as ‘inductance’.

When a spark is required the contact breaker stops the flow of electricity into the primary coil causing the magnetic field to collapse back in towards the coil of wire and this induces a current in the secondary coil.  There are many more windings in the secondary coil winding than the primary and this transforms the original 6-volt supply into the high voltage needed by the spark plug.  The very high voltage created in the secondary winding is sufficient to cause the electric current to arc across the gap between the spark plug electrodes, creating the spark.


Honda C90-Z condensor

The Honda manuals say the condenser (a device that can store electrical energy) has two functions in the system

"the condenser prevents arching at the points and helps to hasten the collapse of the magnetic field...

The first function is explained like this:

As the contact points open, the effect of the collapsing magnetic field in the ignition coil also creates some voltage surge in the primary circuit.  The condenser absorbs the voltage surge and thus helps to prevent the contact points from arching as they separate."

It seems that when the points open the primary coil earth connection to the frame is disconnected but it can now earth through the condenser.  The condenser is able to store enough energy to delay the voltage surge slightly and this allows the points to open far enough to prevent the voltage surge arching across the gap[5].

I could not understand any of the explanations of how the condenser helps hasten the collapse of the magnetic field but apparently this is what it does and is the reason that another symptom of a failed condenser is rough running and misfires.

Contact Breaker

There is more on this critical part in the post on ignition timing, but basically the contact breaker is a cam operated switch that opens at the point a spark is required.  As explained above, when the points open the current flow to the coil ceases and a spark is generated at the spark plug.

Spark plug

Honda recommended a  NGH DR6H spark plug for the C90z models sold in the UK

From the 1978 C90Z user manual (the codes NGK use to describe their spark plug features is described below)

Here is what Honda say about spark plugs:

NGK numbering scheme

NGK spark plug identifiers follow  a numbering scheme that describe the physical properties of the plug.

The code for the UK C90Z spark plug, DR6HS, is broken down as follows:

  • shell dimensions: D (12mm thread, 18mm hex nut)
  • Construction type: R (resistor type)
  • heat range: = 6
  • Thread Reach: H - 12.75mm
  • Firing end construction: S (standard 2.5mm centre electrode)[6]

The older C90s use a conventional 12mm spark plug, but the smaller bikes  use an unusual 10mm plug.    When the 50cc Super Cub was being designed in the late 1950s, the engineers were struggling to get enough power from the engine and wanted to enlarge the intake and exhaust valves but the there was not enough room in the cylinder head when using standard spark plugs.  Rather than expand the capacity of the engine, Honda asked NGK to create a 10mm plug specially for their new bike.  As a result Honda was able to get an output of 4.3 PS for the 50cc engine (about twice as much power as anybody else at the time)[7].  A good example of Soichiro Honda's view that 'common sense is there in order for us to break through it'.

Heat Range

As explained in the Honda manual above, the heat rating is a measure of the ability of the spark plug to dissipate heat.  This is mainly determined by the length of the insulator “nose” that extends into the combustion chamber: a hot plug will have a longer insulator nose that tends to transfer heat slower and this raises the operating temperature of the plug:    

In the NGK numbering system hotter plugs have lower numbers

Because the operating temperature of the plug is determined by a combination of engine design,  riding conditions, engine rpm and load, Honda sometimes specified different heat ratings for the same model in different markets, according to the typical riding conditions, climate and fuel grades that would be encountered.  You will sometimes see in older manuals that two plugs are specified, one  for  normal use and a second, colder, plug  for “heavy stress environments”  (for example extended high speed riding).    

Using the wrong heat range can result in an abnormal combustion process called pre-ignition: the overheated electrode of the spark plug ignites the fuel-air mixture before the spark and this causes the piston to overheat  (when this happens the centre of the piston dome can collapse leaving a tell-tale hole).

Resistor type plugs

This is what NGK says about resistor plugs

At the moment the spark jumps the gap it causes a high frequency burst of energy, known as RFI (radio frequency interference). RFI, as its name suggests, creates static on your radio and interference with other electronic equipment, including the vehicle’s on-board electronic control units (ECUs).

Resistor plugs were developed in the 1960s to suppress some of the spark energy, thus lowering RFI to an acceptable level. Most resistor spark plugs use a monolithic resistor, generally made of graphite and glass materials, to filter the electrical voltage as it passes through the center electrode.

Since resistor type plugs actually “resist” some of the spark energy, non-resistor type plugs actually deliver a more powerful spark.

Although modern TVs and radios are shielded from this kind of RFI, back in the 60s this was a major nuisance and many countries legislated to ensure that car and motorbike ignition systems were fitted with resistors to reduce the interference they caused.  

Honda's original design was to use spark plug caps with built in 5k ohm resistors and non-resisted spark plugs, but in the late seventies/early eighties they specified both resisted spark plugs and resisted spark caps in some markets (including the UK and US), presumably in response to local legislative requirements. Apparently the additional resistance in the high voltage circuit caused by the doubled-up resistors slightly reduces the spark energy but lengthens the time the spark persists.  Some people regard the additional RFI protection as overkill and run non-resistor plugs - while retaining the resistor spark plug caps - on these old battery ignition system bikes with no ill effect, however modern systems are more fussy and might not work well with unresisted plugs if they were not original specified.

Spark Advancer

You can read about the spark advancer in the article on ignition timing

That's it for the ignition system.


1⏎ both static electricity and current electricity involves the movement of electrons between atoms, although in the former case they are transferred because of something called the triboelectric effect. You can read about it here
2⏎ an AC generator that serves as the voltage source for ignition is commonly called a magneto
3⏎ capacitor is another term for condensor
4⏎ actually, as we shall see in a later post, the AC Generator on the C90-Zs will generate just about enough power when in "night time driving mode" to let the bike tick over without a battery, but not enough for it to run smoothly.
5⏎ This prevents the burning and pitting of the points that would otherwise rapidly wear them away. It also explains why excessive sparking at the points is a sign of a failed condenser.
6⏎ there are other variations, for example, A = “special design”; W = “Tungsten” ; Z = Thick 2.9mm centre etc
7⏎ Honda Global