Thanks gtiller i will add that earth today. I run led lights and wondering if that's the cause of the ignition warning light to stay on up to 3k rpm before it starts to flicker and go out. Does the alternator need to work harder generating more power maybe with incandescent bulbs.
ELLIS
It seems that a lot of Brit bike owners [as well as some vintage smaller Asian bikes] think that their Norton, BSA, Triumph, etc. has an alternator that functions like the one in their car. It doesn't. All alternators generate electricity by moving a magnet past coils of wire. Your car & most modern motorcycles have a true alternator. It's rotor [the center part that spins] is an electro-magnet. That means that you have to feed electricity into it, to generate a magnetic field, which then produces more electricity in the stator [the stationary part] as the rotor spins. On Lucas motorcycle systems since aprox 1970, the stator is the part encapsulated in epoxy with wires coming out.
Your car's voltage regulator controls the amount of electricity going INTO the rotor. If the system voltage starts to go to high, the voltage regulator throttles down the amount of voltage going into the rotor. That creates a weaker electro-magnetic field [weaker magnet], hence, less power produced in the stator. When more power is needed [system voltage dropping to low], the voltage regulator increases the power going into the rotor. This makes a stronger electro-magnetic field [stronger magnet]. That stronger magnetic field in the rotor will create more power in the stator. As your car's engine speed varies, or you turn on or off electrical loads [lights, A/C, heater, heated seats & glass, etc] the voltage regulator can compensate by varying the strength of the magnetic field of the rotor.
Our Nortons use what is more properly termed a dynamo. That is because it uses a rotor with permanent magnets. That means that you can not control the rotor's magnetic field's strength, via a voltage regulator, like you do in a true alternator [your car or truck]. The faster you spin the rotor on a dynamo, the more power is produced in the stator [output power].
Since you can't throttle a dynamo's output at the rotor [fixed strength of the magnetic field], you have to find another way to limit the voltage [15 volts max] in the system.
A battery that is not fully charged will use some of the systems' output, to charge itself. The ignition coils & lights will also use some. When the dynamo is generating more power, and therefore higher voltage [high RPM operation] it's possible it will create more power than we need. This situation can occur during conditions where the dynamo is producing more power than we need, such as high engine RPM, with a fully charged battery or turning off your lights. We need a way to absorb the excess that is generated.
Diodes are the electrical equivalent of a one way valve in a hydraulic system, like your home's plumbing system. It lets electricity flow in one direction, but not the other. That's how a diode bridge rectifier works, to turn AC voltage [what alternators & dynamos create] into DC voltage. Our light bulbs don't care if we feed them AC or DC. However, our ignition coils and battery do! Feed AC into a battery and it will die in a few weeks/months.
A Zener diode is a special type of diode. During normal operation, when system voltage is below a designed voltage, for us about 14 volts, it does nothing. It's basically an open circuit [a light bulb that's turned off]. As the system voltage increases above 13.5 volts, it starts to allow electrical current flow in the opposite direction. This causes it to become a load on the system, like a light bulb. It absorbs the excess power to keep the system voltage from going to high. Think of the Zener diode like a powerful light bulb controlled by a rheostat, like dim-able home lighting. It's "dimmer" is voltage sensitive. As system voltage rises towards desired limits, it acts like a dim-able light bulb getting brighter and using more power. The Zener will absorb power as needed to keep the system voltage within the desired limits [15 volts max]. The power it absorbs is disapated as heat. That is why it's mounted in a large chunk of aluminum [your Z plates]. The Z plate acts as a heat sink, shedding that heat, so that the Zener does not fail, due to over heating. This is how a Zener diode(s) control the system voltage in a dynamo charging system, like the Lucas units fitted to our bikes. The downside of a dynamo system, is that at low engine RPM, with the lights on, it may not create enough elecricity to power the lights and charge the battery. This usually happens as the rotor's magnets weaken with age. Using high powered Halogen headlamps makes this situation worse, if you have weak magnets in your rotor.
Grant [gtiller] is correct that the Lucas RM 23 [180 watt] system in the Mk III will lose half it's charging power if one Zener diode fails. Using a full wave [4 diode] bridge rectifier on an RM 23 system , if a diode fails in the rectifier, you only lose 1/4 of the output, per failed rectifier diode. So, using one does offer a benefit. The earlier RM 19 & 21 [120 watt] systems [750 & Mk II Commandos] come with a full wave rectifier, so the above diode failure analysis also holds true, for them. In these systems, a failed Zener diode will usually [not always] cause an over charging situation, because it can not absorb any power. If their Zener fails "shorted", it acts like you have an extra headlamp on high beam, dragging the charging system voltage down, resulting in a low or dead battery.
I hope this helps you understand your charging system a bit better.
Charlie K