A Simple Motorcycle Electronic Ignition Replacement.

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There are many motorcycles which use electronic ignition systems and all have one thing in common, the black box costs a fortune.

For the poor in Britain, or those with small bikes, the costs of replacement can be minimal, as they all have the same basic design. The following simple modification has been run quite happily on Honda NS125, KTM 125 and Yamaha TZR125. All had no electronic ignition systems other than the contents of the generator on the engine. Most wires were burnt flush with the blackened crankcase. This will also work on many larger and smaller machines, with or without wheels.

A little theory.
Four strokes (four cycle, Otto cycle,) engines, when spinning fast, need the spark to happen many degrees before the piston reaches the top of the cylinder. This allows time for the flame front to pass through the fuel and air mixture in the cylinder to be at its full potential as the piston begins to descend on the power stroke. But at low speeds, the flame front still happens at the same speed, so at low revs it is therefore necessary for the spark to happen a little later, so the maximum power in the fuel and air mixture still happens as the piston begins to descend. To allow this to happen, most four stokes have an 'advance and retard' unit, which senses the engine speed and times the spark at the right place according to the engine revs.

The Honda C90, ('stepthru' as its normally called) does not have an advance unit, and because its a small engine with plenty of flywheel, it can run happily without an advance and retard unit.
As many C90's are scrapped each year, there is a glut of C50/70/90 CDI units and these are the core of this monograph.
Many small four strokes can also run happily without an advance and retard unit. But larger four stokes cannot and may damage the piston and crank bearings if the timing is not ideal.

Therefore most four strokes need an advance and retard unit. On early machines, up to around 1985's this is a centrifugal mechanical device, but on modern machines, this is often incorporated in the electronics box. The mechanical designs are usually easy to modify. The latest designs are not easy to do and not discussed here.
So if you do not have a mechanical advance unit, then your machine will have to look elsewhere for a solution. Some possible options are described later.

Two Strokes.
Two stroke (two cycle) engines, because of their design, do not need the spark to advance with increasing engine revs. Tick over on a two stroke works quite happily at the fully advanced setting used for full power, and as such, there is no need to have an advance and retard unit. This makes it very easy to fit almost any electronic ignition unit.

Typical set-up.

This picture shows a C90 rotor with the pulser interrupter as a piece of metal sticking out on the outer edge, which causes the timing pulse at the right place, and the pulser is the black lump between the rotor and stator. Lying flat is the stator plate with the pale lighting and dark CDI generator coils.

There are four or more magnets inside the rotor (it rotates), such that as they pass the stator (static) coils, they induce a changing north - south - north - south alternating magnetic field in the iron plates of the coils, thereby generating electricity in the copper coils of wire.

As can be seen, the four poles at the ends of the coils means that the magnets in the rotors are four, so the N-S-N-S field flows strongly through the iron cores. This is acceptable for a low power machine, but sometimes a little more electrical power is needed in such a small space, so six poles can be used, with six rotating magnets. My old Ducati V twin used about twenty permenanat magnets.

Now we are six. The black and white picture shows another popular arrangement, where the lighting coils are the five coarsely wound coils, while the generator is the obviously different one nearest the viewer. Again the pulser is outside. You will also notice that as a six pole stator, the rotor should have six magnets. You can just make out the size and layout of the six magnets inside the rotor.

Assuming a badly damaged machine.
You may be looking at a burnt out wreck of an engine, I often do, as they are the only ones I can afford.
Remove the rotor. Either use the correct tool called a 'puller', or borrow one and always return it in perfect condition.
If you do not have a puller, then uncrew the central nut on the flywheel until it is flush with the end of the crankshaft.
With a friend, place two tyre levers (tire irons) behind the back of the flywheel, so that they place a pulling force outwards, levered carefully against the crankcase. Hit the central nut sharply with a small hammer until the flywheel loosens. By placing the nut level with the end of the crankshaft, then the hammering will not damage the crankshaft threads. Do not allow the tyre levers to dent the crankcase. A single, firm, sharp tap should suffice to release most flywheels. Remove the nut and flywheel.

Inspect the wiring to each coil and make a note of the colours to each. Locate the fine wound high voltage coil inside the flywheel generator and inspect the components carefully.
Where any wire to the wiring loom is damaged, simply desolder and replace with a new wire. The preferred colours are mentioned below. Ensure the insulating sleeve is securely replaced and will not slide off to expose the bare soldered joint.
I usually remove the backing plate the bike, be it a PE suzuki enduro, TZR road racer, or my own specials, then inspect each coil and if suspect, repair as deemed suitable, taking the opportunity to use the old Honda C90 wring loom to fit new wires exiting from the engine casing.
Eventually you will have nice new wires exiting the engine, an earth wire, and a wire or two from the high voltage coil, a wire or two from the heavier coil for the lighting and battery charging, plus a wire or two from the small pulser unit.
In some cases, the small pulser coil and the high voltage coil may not be earthed, so will have two wires each, one from each end of their coil of copper wire. As these components normally use an earth, then this can be done by extending the wires and terminating them to an earth tag outside the crankcases. If this does not work, then they are probably connected inside the original CDI unit, but for the C90 unit, they can be earthed.

The C90 unit.
The spark from a C90 ignition will jump the gap in a 50cc to a 1500cc bike, as all spark plugs are essentially the same. There is no need to have a big CDI unit for a big bike. (If the cost of four C90 set-ups is one tenth the cost of the genuine article, then at least have a try at fitting four sets.)

The electronic ignition of many motorcycles and similar machines with electronic ignition can be run using the cheapest and most easily available electronic ignition in the world. Honda C90 parts are available from all second hand bike dealers and breakers, usually for a few pounds, always try to get the wiring loom as well, or at least the part of the wiring loom between CDI and engine. The parts needed are the CDI unit, the wiring loom connector and a foot or so of attached wire, and the ignition coil.
I often use the C90 stators inside the flywheel for a supply of suitable copper wire for rewinding this set - up onto other machines.

The simple and commonly available Honda C90 CDi unit works very well with most small two strokes. I can also work with many four strokes which use a mechanical advance and retard unit. To check for a mechanical advance unit, it looks like a set of bob weights restrained by small springs which will fly out when running. This is common on most four strokes with points and can also be a conversion for some older machines. See later.

The picture shows that coils are now very small, some not much bigger than your thumb, by using much higher primary voltages from the electronics box, they can now be placed in the spark plug cap, and each coil triggered from the electronics individually. On the left is the Honda C90 CDI unit, of which thousands are lying forgotten in the back of motorcycle shops. In the middle is a new generation of miniature coil off a small scooter with CDI, although the larger ones will also do the same job. This little set supplies the sparks for my road legsl, NSR125 street luge recumbent and KTM off-roader at a far cheaper replacement cost.
Some equally suitable coils can now be built into the spark plug, which greatly reduces radio frequency pollution problems. Check out the bike and car scrap yards and choose accordingly.

Perhaps you have bought a motocross engine, or have a small burnt out 125 or scooter, and want to see if the engine will run before adding the rest of the wiring. If wanting to attempt this on your machine, then first check the engine has electronic ignition such as looking in a suitable workshop manual, or remove the alternator generator cover and look for the small pulser coil on the outside of the flywheel. Sometimes the pulser may be inside the rotor, so check the manual circuit diagram first.
The diagram should look not unlike the wiring loom in the bottom right hand corner of just one generic wiring diagram for a single cylinder electronic ignition machine from my motorcycle wiring monograph on this website.

Go to you local bike shop and ask for a C90 electronic CDI unit, and the ignition coil with the thick wire to the spark plug, and a length of the wiring loom which attaches to them. As these bikes are scrapped by the millions, also try to get the flywheel and backing plate, (esp. if adding lights to a motocrosser) although this is more of a hassle for the mechanic.
The coil must be one suitable for CDI units, and the standard Honda C90 coil is a small suit sizes lump as shown opposite, but check it has the CDI moulded into it.

You can get the lot for a few quid, under a tenner, as they are all ways surplus to requirements. For this you should be able to get the flywheel, backing plate, a good chunk of the wiring loom which connects to the CDI unit, or if you are prepared to do the work yourself, the whole loom, and the ignition coil with the HT lead.

When fitting this set up on bikes, I just carry the CDI unit with about a foot of wire dangling from the CDI connector, the coil, some spare wire, soldering iron, tape and a multimeter, and perhaps the stator coil should a rewind be needed.

I have yet to know of any CDI unit which has failed. These bikes are usually scrapped because of rust or accident or a badly thrashed engine.

The basic set-up.

The Honda C90 CDI unit is very simple and has only five wires.
The earth, the pulser coil input, the high voltage input, the output to the spark plug coil, and the kill switch wire.
To this will be needed a spark plug coil. Look for the 'CDI' mark on the grey or black plastic coil body. The latest mini coils are also very good, and can be attached directly to the top of the spark plug, with a little modification. Older points coils are not suitable.

The earth. - Green.
This must connect to the engine casing, and to the frame, so the kill switch will also work.

The pulser coil input. - Blue with yellow tracer.
This simply goes to the pulser coil. The change in magnetic field as the flywheel rotates, causes a small trigger spike of electricity to make a thyristor discharge the big capacitor in the CDI unit into the ignition coil. If the timing on a different machine is not correct, then the two wires on the small pulser coil may need to be swapped, so the north-south magnetic pulse is reversed. This allows the trigger to occur at the front or tail end of the pulser strip on the outside of the flywheel. So if your ignition timing is out, try swapping the pulser wires around.

The high voltage input. - Black with red tracer.
This supplies electricity from the stator coil inside the flywheel to charge up the big capacitor inside the CDI unit, so it is ready to be discharged, upon command from the pulser unit.

The output to the spark plug coil. - Black with yellow tracer.
This discharges the energy in the big capacitor into the high tension coil to the spark plug, changing it from a hundred or so volts up to the 20,000 volts or so to jump the plug gap. The ignition coil must be a CDI type, so buy this at the same time as the C90 CDI unit, so they will work well together.

The kill switch wire. - Black with white tracer.
This simply shorts out the power to earth. In the C90, it simply shorts to earth via the ignition key switch, so the pulse from the flywheel high voltage generator does not charge the capacitor, simply preventing the engine from working.

Also refer to the wiring diagram for the position of the wires if the wiring loom connector is not available.

The diagram includes the checking diagram for the CDI unit with a multimeter. The two probes of a multimeter are placed across the various pin connections, and the appropriate reading is checked.
The measurements are in ohm resistance. This is the measurement of the resistance to electric flow in the wire, the higher the reading, the greater the resistance.
Higher resistance may be because the wire is extremely long, or very very thin, or very corroded.

Reading the circuit diagram.
The big circle is the flywheel stator, with the two main coils.
The thick coil supplies the lights and battery.
The finer coil goes to the black/red wire on the CDI unit and supplies about 100 volts AC to charge up the capacitor in the black CDI box.
The small coil on the outside is the pulser and is connected to the blue/yellow CDI wire to send a small pulse as to when the CDI discharges its capacitor across the ignition coil to make a big, fat blue spark at the spark plug.

The two measurements beside the ignition coil are the primary and secondary windings.
Primary is connected to the earth and CDI connector tag, and is the low 0.4 ohm resistance.
The secondary has a larger 4,000 ohm resistance and connects to the earth and spark plug thick HT lead, (without the plug cap, which may also contain a resistor).

Reading the grid box.
In the circuit diagram is a little grid box, with the resistances which can be measured using a cheap multi meter. these are available for a few quid, and available in digital or analogue moving needle types. Either will do. Place the probes across each connection and measure the readings. Fairly close will usually suffice, as such devices either work or not.

The open ended figure eight on it's side is infinity, so maximum resistance with effectively no connection is required.
SW is the switch , which is the black and white kill-switch wire.
PC is pulser coil.
E. is the earth.
IGN. is the ignition coil.

The modification.

The backing plate can now be removed for inspection. If the securing screws are in slits, then make sure you mark a scratch on crankcase and backing plate to preserve the correct position.
If the wiring is badly damaged, then check what each wire does by checking with the original wiring diagram. This is often at the back of the owners manual supplied with each machine, and does not need the expense of the larger workshop manual.
If damaged, remove each coil and attach new connecting wires to each coil, in the same way as the original.

If the lacquer on the copper coils is damaged, it can be cleaned up and re-lacquered with nail varnish. If the coils are badly broken or suspect, they can sometimes be wound back a few turns and rebuilt using more lacquer and wrapping in masking tape. Vinyl tape is not strong enough in the hot engine area. The best wrap is cotton cloth and epoxy resin.

If badly damaged, such as sliding down a road and rubbing away much of the winding, then carefully unwind the copper wire, noting the direction of rotation and counting the number of turns. Play it out on the floor, then buy new lacquered copper wire of the same diameter and rewind the same number of turns. Fit new connecting wires to the loom. It is not rocket science. A roll of copper wire costs just a few quid.

I most cases, I simply need to tidy up the windings, and solder in new wires for neatness.

Replace all components ensuring the wires pass through the waterproof seal. If this is missing, then silicone bathroom or gasket sealer is acceptable.
Wire up appropriately as mentioned earlier and check for a spark.

Yeah , but ,

Other stuff.

If you have a twin cylinder machines, then the CDI cannot handle two plugs, so use two CDI units triggered off the single pulser coil. If two pulser coils, then this is simply a case of two separate systems. In many cases, both CDI units can be powered by the single generator coil.

Not everyone has an easy bike.

If you have no advance and retard unit on your four stroke, then the electronic ignition just does this job and building your own is going to be difficult or expensive, so simply buy the correct CDI unit.

Do NOT Do This:
If you have a four stoke engine and manage to wind a higher voltage CDI charging coil in the flywheel generator and can get a spark from it using the other components, (perhaps sticking a old fridge magnet to the flywheel to get the pulser to work) while cranking over and checking for a spark, then you may need an advance and retard unit.
If wanting to make your own advance and retard unit, perhaps for an older bike, extend the camshaft, by drilling or welding a shaft or stud into the end of the camshaft, and extending it out though an oil seal.
Most camshafts will need drilling centrally, so remove any end plug, and pack some cloth around the camshaft oil area. Fit a top quality drill in the drill and start the bike engine, then slowly drill a perfectly concentric hole in the end of the rotating camshaft as the engine ticks over. Stop the engine, clean up the swarf, and either tap the hole with a decent thread to take an extending bolt and sleeve, or hammer in a tight fitting smooth steel rod, preferably with some epoxy resin and roughen the end of the rod where it fits into the camshaft.
Rotate the engine on the electric start and make sure the new shaft is perfectly concentric, or gently hammer so it runs true. Place an oil seal around the shaft and fit a smpth steel tube of suitabel dimensions to fit the oil seal. Secure the seal in poition by building up the casing to take the seal with 'epoxy metal in a tube'. Make sure the camshaft casings will split where needed, so you may need to fit slivers of cooking cling film to make a splittable seal joint face.
Fit the CDI pulser and advance unit from an eighties bike such as an XL125 then build up the camshaft area with epoxy metal to make a simple cover and to hold the pulser in position beside the rotating advance retard unit. This shaft will probably need a slot or hole to align the advance retard unit, and retaining screw. Rotate the engine to the static firing mark, F, then align the advance unit and the pulser so they line up. There is often manufacturers marks for this.
If a four cylinder, then you'll be adding a second pulser coil 180 degrees apart, or if a V4, then at 90 degrees or whatever is needed.
Then check the spark advances as required, or be prepared to bend the centrifugal weight end stops to get perfect advance position, then adjust the springs to get the best advance curve

Adding lights to motocrossers and mini motos.
Open up the flywheel generator and if there is only one winding, then buy another, which will fit on the other side. This can be wound with fewer turns of thicker enamelled copper wire, and I would recommend buying an old C90 lighting winding as it will have the right amount of correct copper wire for a few quid. Unwind it carefully and then rewind it onto the stator coil of your machine.

If you can't buy a new coil, then you can try winding the lighting coil over the CDI generator coil, but due to the room available, just don't expect too much. If desperate, then you can try for a 6 volt lighting system with small wattage headlight and LED tail lights.

A recent CDI custom special of mine needed to retain the '12volt' cdi system, but preferred a 6v lighting system to keep weight to an absolute minimum, by using small NiMh batteries in the frame tube to power just the indicators and horn. The lighting battery generator coil was carefully unwound and the wire folded half way, with an earth tag fitted half way along the wire. This was then rewound as a pair of wires and used in 6 volt mode. The reason why this was possible is that only half the number of turns was available on each wire, causing only half the voltage. But as the wire was doubled, the output of the 6volt wires was therefore double, allowing the same wattage lights etc, but at 6 volts rather than 12 volts.
Likewise a 6 volt lighting coil can be rewound to supply 12volts simply by halving the cross section (not the diameter) of the wire and doubling the number of turns.

Modifying motocross or trials machines for road use is common. I've fitted a Honda 90 flywheel and backing plate with spark coil and lighting coil onto a Suzuki PE175 crank flange using bolts as rivets, then timed it correctly using C90 cdi components. Make sure the positioning of the pulser is correct before final fitting. This modification started first time and was still working after four years. A few welds to the original Suzuki crank mounting taper flange would probably make it even more reliable.

If making a Dakkar or night desert racer, then cram in as much winding as possible, - same number of turns, but thicker wire and add some over the other coil too, then fit a decent rectifier regulator unit with big fins.
You can now add a headlight and tail light, with no other items, but a switch is common. If using high and low switch, always make sure the tail light is switched off at the same time as the switch over, otherwise it may blow. Alternatively use a 12 volt regulator in the tail light wire to protect it. Lights can work directly off the AC electricity, so no fancy gubbins are needed, - just make sure your headlight is big enough to take the electricity at top revs without blowing. This is called a 'balanced system' and is simple, but if the headlight blows, it also takes out the tail light, so carry both as spares. LED tail lights ar more robust.
If wanting a battery, then read my monograph on motorcycle electrics, it's on this website

Points ignition.
If your machine has a set of points, then it either uses 12 volt battery ignition or uses direct pseudo magneto ignition. Whether 6 or 12 volt battery points, or the magneto version, then both can have the coil which supplied the HT coil rewound to supply about 100 volts, - rewind using an old C90 winding, or similar, then test using a multimeter when kick starting. Then position the piston at the correct timing and fit the pulser and small magnet on the outside of the flywheel. If necessary, make a hole in the alloy casing and epoxy a pulser in place, modifying the timing by repositioning the magnet.
If an advance and retard unit is used, then the unit will have to have a small magnet epoxied on the unit, and the new pulser positioned with an airgap of 5 thou. The pulser will have to be axially adjustable to get the timing accurate.

E-mail for alternative ways to solving a wide variety of engineering problems. Well, almost any.

If doing this modification, then also read 'Stonehenge and Spanners', on this website, which is aimed at home motorcycle builders and engineers. Reading this can save a lot of grief.

The lighting coil. This is not part of the spark circuit, but as the bike may be burnt out, then after it runs, you may as well get the lights working too.
The larger lighting and battery charging coil which has thicker copper wire windings, may have two outputs, usually one for charging the battery and the other for direct lighting. To find out which is which, simply place a headlight to each, to see which is which lights the headlight bulb the brightest when the engine is running. The lesser wire is to charge up the battery via a diode. If you have a multimeter, then the wire with the higher resistance is the lighting coil, and the wire with lower resistance is used for charging the battery.
Headlights can work quite happily on AC, so no fancy electronics is needed. If the direct lighting keeps blowing, simply fit larger wattage bulbs or add a simple 6 or 12 volt limiter to the headlight circuit. A limiter is a cheap and basic device which simply bleeds off excess voltage, so the bulbs do not blow. These are often found on small commuter bikes and sell second hand for a few quid or new pattern parts cost about the same. They are simple single blocks attached to the frame, with a tag for the lighting wire and will bleed off excess electricity. Just connect it to the tail light wire so this will remain reliable. Never fit it to the wire from the generator, as this will cause it to blow. It must always be connected after the lights switch.

For charging the battery, use a single diode. Any diode will do and a 50 p diode which can handle 4 amps can be soldered in the line to the battery, but make sure its the correct way around, with the + end to the battery. Diodes often look like little black cylinders on thick wire, with a silver band on one end. You can scavenge these diodes in groups of four from the power supplies of old video recorders and TV sets etc.
For greater efficiency, or if riding at night a lot or prone to stopping with your brake light on all the time, or using indicators too often, then isolate both ends of the lighting coil wire and use four diodes in diamond arrangement (Bridge rectifier) to get both halves of the AC current going to the battery. See the motorcycle wiring monograph on my web page.
The animation shows how both directions of the yellow alternating current is used to re-direct it to become the red positive and green negative DC direct current.

If revving heavily on a regular basis, then a voltage regulator will keep the battery from boiling.

Best wishes,

John Partridge. B.Ed. B.Sc. etc.

I am looking for work.
Gizzajob.

Companion monographs available on my website.

A Builders Guide to Motorcycle and Trike Wiring. *****

A Beginners Guide to Motorcycle Mechnics Basic.
A Beginners Guide to Motorcycle Mechnics Intermediate.
A Beginners Guide to Motorcycle Mechnics Advanced.

A Builders Guide to Trike Design. *****
A Builders Guide to Motorcycle Design. ******
A Builders Guide to Composite HPV Cycle Design.
A Builders Guide to Composite Motorcycle Design.
A Builders Guide to Campervan Design.

A Beginners Guide to Build your own Computer. *****
A Beginners Guide to Build your own Stair Lift.

A Beginners Guide to Make your own glasses. (spectacles.)
A Tourist Guide to Plymouth. Barbican/Hoe, no tourist spin.
Domestic repair, maintenance. water electric gas house cars.

A Builders Guide to Motorcycle, Trike and Car Wiring ******
A Builders Guide to Camper Van Design
A Builders Guide to Aerodynamics and Wind Tunnel Design.

A Beginners Guide to The Outdoors. How to walk.
A Beginners Guide to Survival Knife Design.
A Beginners Guide to Survival Kit Design.

A Beginners Guide to Teach Yourself to Swim.
A Beginners Guide to Look after Your Teeth.
A Beginners Guide to R/C Gliders. An easy way.

A Beginners Guide to Build your own Steam engine.
Simple Harley rewire. A lifestyle thing.

Stonehenge and spanners. ****
Easy life. ****

Always try to improve society rather than just take from it. Until then, lawyer stuff. All material herein is subject to copyright, patent and other intellectual property rights. Modification of this material whole or in part is prohibited without the written permission of the author. Contents subject to change without notice. Errors and omissions excepted. Contents for illustrative purposes only. All rights reserved. No responsibility is accepted for any damage or any injury caused by this information. No-one should try building equipment without reasonable abilities and know that injuries can ensue from the materials, tools and from testing and use.

Copyright (C) J.Partridge. B.Ed. B.Sc. 1993. 2002.