What is a CDI box? How does it work? How come some vintage motorcycles use it while others don't?
When looking at this from the general guise of nearly all vintage motorcycle CDI (Capacitive Discharge) ignition systems, excluding the newer advent of computer controlled ECM's (engine control module and EFI (electronic fuel injection) systems. All this little box is doing is storing and controlling when an electrical charge gets sent to the coil. Technically speaking this electrical charge gets sent to the primary coil inside of the high tension coil. What? In layman's terms the high tension coil is the spark plug coil, or "coil" unit that the spark plug wire is attached to.
Not to get side tracked, but on the internet and discussion boards the term "coil" is often used very loosely. It is usually used in the context of the aforementioned spark plug coil. To confuse matters more, this "coil" actually contains two different coils inside of it, a primary & secondary coil, working in conjunction to step up the voltage to jump the spark plug gap.
The truth of the matter is, in vintage motorcycle ignition systems (no matter the type) there are multiple coils, found in different places, that serve different functions, for different things. In the most generic term a coil is an electric circuit, one or more turns, usually roughly circular or cylindrical, of current-carrying wire designed to produce a magnetic field or to provide electrical resistance or inductance. source
Alright, enough, weren't we talking about the mysterious CDI box?
In the most simplest terms a vintage CDI box is a collection of diode's, Silicon controlled rectifier (an electrically controlled gate), and a capacitor. All these solid state components wired in together in a specific arrangement serve to replicate and replace what was (and still is in simple single cylinder engines) a mechanical means of timing how & when the spark plug(s) need to fire. This mechanical type of ignition is commonly known as a contact breaker point or "points" ignition.
To better understand CDI ignitions it would be wise to explain how a points type ignition works as the CD is electrically replicating what the points are doing.
Before we can talk about the breaker points themselves, we have to understand where the electrical charge that eventually makes its way to the spark plug gap originates.
*Origin of a spark*
If you remember your high school physics class where you created a simple electro magnet by wrapping a wire around a nail and using a battery you could create a magnetic field to pickup a pile of paper clips. In a simple motorcycle ignition the same principles are being used, except instead of a battery, the flywheel contains magnets that fly around a stationary coil of wire wrapped around a iron bar. The passing of the magnetic field by the coil of wires induces a small electrical current. That current is then sent on to the ignition system as a whole.
*Breaker Points*
First off the breaker points are central to the whole system. They are the mechanical means for controlling the timing. They are often found in one of two places. On 2 stroke engines they are found behind the flywheel, attached to the stator plate. On 4 strokes they can also be found attached to a plate that is at the end of the camshaft behind a cover.
If you are new to this sort of thing, a service manual is a must to learn where on your particular engine to find the breaker points, because they require periodic maintenance.
How do the breaker points control ignition timing? Simple, they are fixed to an adjustable plate, they contain a spring and a phenolic plastic block that rides against a cam that is directly connected to either the crankshaft OR camshaft (on some 4 stroke models). The cam is keyed to crankshaft or camshaft rotation and has a hump to control the open, close and duration of the breaker points as it rotates. An adjustment slot where the breaker points mount allows for small adjustments for the gap when the points are open. Also the plate the points are mounted to have adjustment fore/aft to adjust when the points begin to open.
And here lies the major Achilles heal of a breaker point ignition, there are wear items that degrade with normal use. The plastic wear block, wears, so constant maintenance of the point gap needs to be done to account for this wear. Also the contacts wear away from the snapping open every revolution, which also continuously alters the gap between the points. How often would you need to do this maintenance? Well depending on riding habits and operation this could need to be done every few thousand miles with replacement intervals generally around 5 to 6 thousand miles.
Next to the points, and I mean literally mounted next to the points is a part called the condenser. A condenser is a capacitor just of a different name. They function electrically the same. It's basic function is to be a temporary battery or storage device for that electrical charge that eventually gets sent to the high tension coil for amplification. From a maintenance standpoint a condenser has a longer service life over the breaker points, but they aren't fail proof. A condensers construction has two metal plates that are separated by a dielectric insulation or compound. This insulation is susceptible to temperature degradation over time. Thusly its performance is affected by temperature, this doesn't help that it has to be mounted right near the heat source of the engine. Typically people replace the condenser at the time they replace the points as it is no more effort to do so.
For a seasoned mechanically inclined owner, this maintenance is cathartic, but to someone trying to use these bikes as purely an appliance, they just want them to "run" and not have to think about it. With the advent of a CD ignition this was a big sell in marketing materials of the time. Lots of advertisements called it a "maintenance free" ignition.
So what parts are analogous from a breaker point ignition in a CDI ignition?
When looking at this from the general guise of nearly all vintage motorcycle CDI (Capacitive Discharge) ignition systems, excluding the newer advent of computer controlled ECM's (engine control module and EFI (electronic fuel injection) systems. All this little box is doing is storing and controlling when an electrical charge gets sent to the coil. Technically speaking this electrical charge gets sent to the primary coil inside of the high tension coil. What? In layman's terms the high tension coil is the spark plug coil, or "coil" unit that the spark plug wire is attached to.
Not to get side tracked, but on the internet and discussion boards the term "coil" is often used very loosely. It is usually used in the context of the aforementioned spark plug coil. To confuse matters more, this "coil" actually contains two different coils inside of it, a primary & secondary coil, working in conjunction to step up the voltage to jump the spark plug gap.
The truth of the matter is, in vintage motorcycle ignition systems (no matter the type) there are multiple coils, found in different places, that serve different functions, for different things. In the most generic term a coil is an electric circuit, one or more turns, usually roughly circular or cylindrical, of current-carrying wire designed to produce a magnetic field or to provide electrical resistance or inductance. source
Alright, enough, weren't we talking about the mysterious CDI box?
In the most simplest terms a vintage CDI box is a collection of diode's, Silicon controlled rectifier (an electrically controlled gate), and a capacitor. All these solid state components wired in together in a specific arrangement serve to replicate and replace what was (and still is in simple single cylinder engines) a mechanical means of timing how & when the spark plug(s) need to fire. This mechanical type of ignition is commonly known as a contact breaker point or "points" ignition.
To better understand CDI ignitions it would be wise to explain how a points type ignition works as the CD is electrically replicating what the points are doing.
Before we can talk about the breaker points themselves, we have to understand where the electrical charge that eventually makes its way to the spark plug gap originates.
*Origin of a spark*
If you remember your high school physics class where you created a simple electro magnet by wrapping a wire around a nail and using a battery you could create a magnetic field to pickup a pile of paper clips. In a simple motorcycle ignition the same principles are being used, except instead of a battery, the flywheel contains magnets that fly around a stationary coil of wire wrapped around a iron bar. The passing of the magnetic field by the coil of wires induces a small electrical current. That current is then sent on to the ignition system as a whole.
*Breaker Points*
First off the breaker points are central to the whole system. They are the mechanical means for controlling the timing. They are often found in one of two places. On 2 stroke engines they are found behind the flywheel, attached to the stator plate. On 4 strokes they can also be found attached to a plate that is at the end of the camshaft behind a cover.
If you are new to this sort of thing, a service manual is a must to learn where on your particular engine to find the breaker points, because they require periodic maintenance.
How do the breaker points control ignition timing? Simple, they are fixed to an adjustable plate, they contain a spring and a phenolic plastic block that rides against a cam that is directly connected to either the crankshaft OR camshaft (on some 4 stroke models). The cam is keyed to crankshaft or camshaft rotation and has a hump to control the open, close and duration of the breaker points as it rotates. An adjustment slot where the breaker points mount allows for small adjustments for the gap when the points are open. Also the plate the points are mounted to have adjustment fore/aft to adjust when the points begin to open.
And here lies the major Achilles heal of a breaker point ignition, there are wear items that degrade with normal use. The plastic wear block, wears, so constant maintenance of the point gap needs to be done to account for this wear. Also the contacts wear away from the snapping open every revolution, which also continuously alters the gap between the points. How often would you need to do this maintenance? Well depending on riding habits and operation this could need to be done every few thousand miles with replacement intervals generally around 5 to 6 thousand miles.
Next to the points, and I mean literally mounted next to the points is a part called the condenser. A condenser is a capacitor just of a different name. They function electrically the same. It's basic function is to be a temporary battery or storage device for that electrical charge that eventually gets sent to the high tension coil for amplification. From a maintenance standpoint a condenser has a longer service life over the breaker points, but they aren't fail proof. A condensers construction has two metal plates that are separated by a dielectric insulation or compound. This insulation is susceptible to temperature degradation over time. Thusly its performance is affected by temperature, this doesn't help that it has to be mounted right near the heat source of the engine. Typically people replace the condenser at the time they replace the points as it is no more effort to do so.
For a seasoned mechanically inclined owner, this maintenance is cathartic, but to someone trying to use these bikes as purely an appliance, they just want them to "run" and not have to think about it. With the advent of a CD ignition this was a big sell in marketing materials of the time. Lots of advertisements called it a "maintenance free" ignition.
So what parts are analogous from a breaker point ignition in a CDI ignition?