British Motorcycle Electrical Systems
THE DARK SIDE for many. Never fear, this won't make you an expert (X is an unknown quantity - Spurt is a drip under pressure) so be careful who you call an expert. In the classic bike field there are plenty. Nevertheless, read on if you would like to have a bit more understanding of your machine. If you have a better than factory perfectly restored classic and don't ride it then you won't need this info.
Don't knock Lucas too much: sure there were problems with some of their products but we are talking about stuff made many moons ago. When it was new it wasn't too bad. Obviously what is available today is streets ahead so no comparison should be made. Over the years I've found a lot of Lucas problems were actually attributed to owners whose knuckles dragged on the ground. Self imposed electrical carnage, such as leaving bare wires to short; taping up twisted and unsoldered joints; putting foil in the fuse or no fuse. Fuses blow for a reason, not usually through age. Things would be easier if a small 3 or 4 panel fuse box were installed but they weren't so either add one next time or deal with a single inline type that most makers deem fit. If the fuse is blowing without even turning on the key switch then look at the old zener diode or wafer rectifier, try removing the connections from one device at a time, (the horn feed is usually the only other thing directly connected to the battery before the key switch), pull the wires off it and try a new fuse. If it is still blowing with these 3 things disconnected look at the main harness for signs of heat ie wires melted together. By the way if the bike has no power or lights etc, or stops suddenly then it may be the ammeter, especially some Asian repops, they can go open circuit, no electron flow. Remove the headlight and bridge the terminals on the meter or put all the connections together. It will get you home. If the fuse blows with the key on, check the lights. If they are off, check the feeds at places like brake switches, coils and in the headlight shell for shorts. If the fuse blows when using the brake then check under the rear mudguard where you may may find frayed shorting wires. This may be due to the rear tyre rubbing because of too big a tyre or too short a shock, or too lard an ass, in which case use a longer shock. If the fuse blows when the lights go on then again check the tail loom under the rear mudguard, or the hi/lo handlebar switch. That can be a problem. If not, then into the headlight, check the wiring and plug socket on the semi sealed beam: they can be a pain. Separate the front loom from the tail loom under the seat when testing. It's a process of elimination.
Your charging will need some sort of an upgrade at some time. The main culprits, the Zener, the wafer bridge rectifier, and the rotating magnet (rotor) This is 1 of 2 parts that make up the alternator, the other being the stationary winding (stator). The rotor has 2 problems, one being reduced magnetism with age and the other being a loose centre. This happens because it has separate pole material from the centre that goes on the crankshaft and then they're cast together. Over time it starts to break up and can be heard as a knocking sound in the primary. Worst case, it can come apart and weld to the stator and twist the crank end. High output stators are available in 180-200 watt or standard output 120 watt single phase or 200 watt 3 phase types as introduced with the 1979 T140. New rotors are also a welded type to stop disintegration. There are a few makers and they all seem to be fine. With the new alternators your best advice is to upgrade to a combination electronic rectifier/regulator made to cope with outputs of modern alternators: make sure you get the correct combination. The new regulator/rectifier allows you to wire positive or negative ground/earth. This can all be put in some early machines such as pre-unit Triumphs. If you are rewiring your machine and not using a replacement loom, you will need at least an 18 gauge wire. Remember at 12 volts, a headlight of 60 watts will use 5 amps. The main loom wires from the headlight shell (ammeter) to the battery area need to be the biggest. This includes the ground wire which needs to be all the way from the battery onto the frame then to the headlight. Don't rely on the steering head bearings for a ground. Make off the terminals with a decent crimping tool and seal them with heat shrink: that will keep moisture out of the joint and help to stop fracturing. Soldering the joint is good too but even though I've done both, I don't have a preference. Always leave a bit more than you need. A tight fit at the device will mean a broken wire sooner or later. Don't join wires in the loom under the taped wires and especially don't change colours under the tape or heat shrink. Use as many colours as you can afford, not just one.
You may wish to go to an electronic ignition. These devices have become very reliable in the last 15 yrs and there at least 4 makers of units for Twins. Triples and Singles. Remember the original analogue types do use more current and are therefore more voltage sensitive. Upgrade or at least know your charging system's good before going electronic ignition. If you develop a backfire after riding a while with the headlight on, turn the light off and if the bike starts to run better then the charge output is low and you probably need a rotor. Analogues will use up to 4 amps or 48 or so watts. If you have an original 120 watt system (at around 4000rpm) you can see where with the head light on, using 60 watts+, how it will overcome the output of a weak original system around town at lower rpms. The digital units may be better as they are low wattage users, although they are quite a bit more expensive. With a strong charge, analogue is fine. If you have an older magneto ignition machine, you can nowadays upgrade to an electronic ignition that directly replaces it. If you want to ride a lot, it's the only way.
You may also decide to upgrade the old generator to an alternator replacement. Pazon and Alton are a couple of names that produce these conversions. Definitely worthwhile. I have an old pre/unit set up with an electronic ignition and Alton alternator. Some machines even have electric starts being made for them. Modern age oldie. If you decide to go batteryless then make sure you have a regulator/rectifier with a capacitor incorporated in its design. This can also be done with a capacitor from the Norton commando models. You just need to have some way to smooth out the AC ripple. The battery normally does this job. The rectifier doesn't quite produce a dead flat DC wave instead it leaves a minor ripple. This interferes with the ignition, points or electronic. The machine may start but won't rev much off idle, so turn on the headlight: with the heavy load of the light filament it will. A good charge is needed to prevent stalling when idling with the lights on. Stay with points or go digital.