I'm getting quite a few inquiries where the writers are confused about how the ignitions work on these bikes. It's probably due to many new members being too young to remember/use cars with Kettering ignitions (e.g., points, condensors and coils), so here's an explanation of how these systems work, for everyone:
- Most of these bikes (the CB750/500/550/400F/350F) use conventional Kettering points, condensors, and coils type ignition.
- The CB650 bikes use a modified version of 1960s-era CDI (Capacitive Discharge Ignition) all-electronic ignition.
- These 2 systems work very differently.
The Kettering systems use points that close to pass power through the coil (which is connected to battery power) to build up a magnetic field in that coil's metal core, and at the desired moment of spark (timing) these points are bumped open by a small cam (on the spark advancer), which causes the magnetic field to collapse suddenly. This collapse causes large magnetic flux across the coil's windings, resulting in momentary high voltage at the sparkplug. The condensor, which is connected across the points, absorbs the coil's angry backlash at this event, which is about 400 volts, and stores it up for the next time the points close again. Then the cycle repeats as the condensor's power is dumped to ground when the points close, and the charging cycle starts over. A little-known, but important, feature of the condensor is: its value (0.24uF, 600v) is tuned to match the inductance of the coil, which helps peak the coil voltage during spark. This value also helps to extend the duration (time) of the spark from the coil, and helps reduce the arcing that happens when the points open to interrupt the charging current. Honda engines like these need about 4000 volts of spark to run, and the Honda coils develop about 7500 volts for about 1.2mS, plenty of extra margin there.
The [original] CDI systems of the 1960s car-era racing designs used an oscillator and transformer to develop high voltage (usually between 200v and 600v, depending on the design) from the car's 12 volt system. This power charged up a capacitor (condensor) that was dumped into the coil at the moment of timing, which created a much hotter spark of about 50,000 volts, although it is very short in duration - like 0.5mS to 0.8mS duration. These systems use upwards of 8 amps of power to make this spark (some used up to 22 amps!). For car engines this was OK. Motorycles do not make much electrical power, but Honda wanted CDI-like ignition for the CB650 engine. So, they (TOYO, actually) developed a less power-hungry CDI which leaves the coils OFF until the moment of spark, then dumps a very fast-turn-on switched current to the coil when spark is needed. Then it immediately switches back off again to save battery power. To do this, they suspend (bias) the trigger coil on the timing plate between 5 volts and 12 volts of power (for a difference of about 7 volts) and swing a trigger past the coil to disturb the magnetic field that this current creates in the coil. This 'pulse' is resisted (electrically) by a capacitor (condensor) in the electronic circuit for about 1.0mS, during which time the whole rest of the circuit board turns on the coil. This sudden spike of current charges the coil until this condensor 'times out' the pulse circuit, and then the coil collapses to make a spark. It is a short, hot spark of duration set by the trigger capacitor (condensor) and is cleverly created to use less power than even the Kettering designs of the other SOHC4 bikes, because the coil only eats current during the very short charging spike. These coils are very low resistance so they can 'move' this fast, so they are not suitable for the earlier Kettering designs: installing them onto the Kettering bikes can melt the coils and their wiring.
This CDI design persisted in many Japanese bikes for decades afterward, as it is highly successful, so long as the electronics live.
In the 1970s, several "electronic" systems appeared for these, and other, Kettering-fired bikes. The most common one is the Dyna S type, where a magnet on the points cam momentarily turns OFF a Hall-Effect switch on the trigger board (one for each coil, 2 circuits per bike). These Hall-Effect switches turn OFF when a specific magnetic field is passed next to them: in the Dyna system they stay ON until Dyna's magnet, mounted on the [modified] points cam, swings past them. This ON state simulates the Kettering points being closed to charge the coils. The fly in this ointment is: they are ON all the time until the magnet swings part them, so the coils are drawing current all that time. Since these bikes have alternators that were only sized to deal with 1 coil charging up at a time, the coil electrical load is now doubled. This drains a lot of extra power from the bike. The 500/550 has the biggest trouble with this system, as its alternator rotor is poorly made and the alternators do not make much power as the result. Even the 750 has trouble supplying enough power for this unless it is ridden on the hiway at speeds over 4000 RPM for most of the ride time.
The Tytronics system is very much like the Dyna system. Newer versions of this one are reputed to have 'dwell limiters' in them to reduce the ON time somewhat, but the jury is still out as to how well this works, and its longevity, given where it is made.
Boyer-Bransden made an optical version a long time ago that was a compromise in power consumption between the Kettering and Dyna designs. Today this system seems to be available in UK on-and-off, and may come to the rescue of those who do not wish to live with points at all. It uses optical switches instead of Hall-Effect devices, with timing circuitry added to reduce over-consumption of coil charging power time duration.
My own Transistor Ignition merely uses the existing points to switch transistors instead of coils, in order to remove the arcing that wears the points. Since most of the points foot wear is caused by the points arc-welding themselves shut after each spark above 5500 RPM, this box stops that wear completely. This prevents point-foot wear almost 100%: I ran 9 years with my own unit #2 (Terry in AUS has #1) with no timing adjustments needed until I tore the whole engine down for rebuild. Then it was reset in 2013, hasn't needed anything since. I first developed this in 1973 to reduce the constant need to adjust points then, although it was so expensive compared to points then ($140 USD back when points cost $3, and in those $$ it would be about $280 today) that few of them got installed. Today, this is a different game, and it also supplies 100% "backup" in the form of being able to plug your points right back in if somehow the box should die. Thus, you can still ride home instead of hitching a ride to the nearest pickup truck or trailer. In America, where it is more than 3000 miles shore-to-shore and my average tour mileage was 2x to 3x that much mileage, this is a useful feature.
