Although there is a rich tradition of the electromagnet-powered charging system's use in motor vehicles, its application in 1970s four cylinder Honda motorcycles is unique. The electromagnet system's beginning reaches back to the DC generator, but we'll not go back that far. Two things about SOHC four charging systems demand that they be troubleshooted in ways very unlike the electromagnet based system on your car. First, the Honda fours' systems are modular as opposed to integral as on your car. Consequently there are an abundance of parts as well as dozens of connectors scattered all over the motorcycle. Second, unlike a car's alternator, your Honda's charging system runs at crankshaft speed, meaning it does not benefit from pulley ratios, particularly at low speed, so that its performance curve is very dissimilar to that in car systems. SOHC fours do not charge at idle. These differences have confused and misled many a DIYer who assumes familiarity.

How the system works
Even before the engins is started, the electromagnet based alternator is working. When the keyswitch is turned on but the engine not yet started, battery power enters the electromagnet, producing magnetism at full strength. Once the engine is running, this magnetism induces voltsge in the stator which moves electrical current. The stator's output goes through the rectifier to be converted from AC to DC--the AC produced by the revolving electromagnets’ two poles is not compatible with the battery. The battery is charged by this “rectified” (redirected) current. Since more engine rpm will unendingly produce more system output, the regulator waits for the presence of 14.5VDC at the battery indicating it is fully charged at which point it weakens the electromaget by interrupting its feed. That is until battery voltage drops below 14.5VDC at which point the regulator strengthens the electromagnet, allowing charging to resume. Whether by vibrating points as found on 69-74 SOHC models, or solid state as on later SOHCs--and whether stand alone or inside a box with the rectifier--the regulator caps off total chsrge by controlling magnetism. Unlike the permanent magnet alternator's regulator which works in an on/off fashion, the electromagnet alternator's regulator functions in a tapering up and down manner, making it a bit more sophisticated and less electrically abrupt.

Switches, wiring and connectors
Before troubleshooting look closely at the wiring and connectors. They are often overlooked, and they are responsible for at least eighty percent of the electrical problems on vintage Hondas, and in many cases 100 percent. Any connectors that show melting of their plastic housings (“canon plugs”) must be replaced. There are several good sources of these parts online, including the brass terminals inside them. A special tool is needed to disassemble canon plugs, available all over, but a good substitute can be made from a piece of hacksaw blade or feeler gauge. The terminals will inevitably be at least lightly corroded. Wire brush them, then smear a little soldering paste over the crimp, and using good electronic (60/40 ratio, rosin-core) solder, lightly and carefully solder them. And don’t overlook the mainfuse holder or fusebox. Again, visually inspect, de-corrode, and solder.

Regulator
There are seven common issues with vintage Honda elecromagnet charging systems. The first concerns the point type regulator, which can wear out its points, Second, it can even have points that vibrate out of sync with the regulator's instructions due to excess vehicle vibration. Honda issued a service bulletin on this latter problem. Third, the solid state regulators can burn out, usually due to a short in the electromagnet which demands too mich current to go through the regularor. Remember, the regulator in the electromagnet alternator is in the electromagnet's feed path. As with any charging system, the best way to test the regulator is to bypass it. On the points type, simly jump over the black and white terminals using a jumper wire, and retest charge. On the solid state type, jumper from the white wire in the canon plug to the battery negative terminal, and retest charge. If charge increases dramatically, the regulator is the problem.

Keyswitch
The fourth issue that can come up is that certain models of keyswitches build up resistance internally at their contacts. When this happens, the electromagnet charging system will under-produce—the bike will lose charge. This is because the keyswitch is part of the charging system: it is in the circuit that feeds the electromagnet. Poor keyswitch contacts result in low magnetism.

Overloading
The fifth problem on vintage Honda twins is when a non-original headlight or ignition coil is fittted. Any increase in the headlight wattage will greatly affect charging. The same for an aftermarket ignition coil. A coil whose primary resistance is less than 2.2 ohms will impose so much of an electrical load that it will significantly reduce charging to the battery.

Rotor
Problem number six is a weak electromagnet. A number of things can cause this, including bad connectors and worn brushes (SOHC 650 four). Problem number seven was alluded to above, namely a shorted electromagnet. This too is commn on the SOHC 650 four.

Differences
There are two variations on the electromagnet alternator. Early ones use a third alternator part called a reluctor interposed between the rotor and the stator. This eliminates the need for brushes. Later electromagnet alternators use brushes instead. Early electromagnet alternators use point type regulators wired into the electromagnet's feed on the positive side. Later electromagnet alternator regulators are solid state wired on the electromagnet's negative side. Both do the same job of raising and lowering the electromagnet's magnetic strength.

Testing the battery
After the wiring and connectors inspection and consideration for known issues, it is imperative to charge and load-test the battery. The battery affects the charging system in ways that are not intuitive. It can also be failing completely on its own. Charge the battery with a battery charger having at least a 2-amp output. “Tenders” or trickle chargers are not adequate for this. Determine sufficient charge by a combination of open terminal voltage and on flooded-cell batteries a specific gravity reading of the electrolyte. “Open” means nothing connected. The battery is fully charged when the open terminal voltage is 12.7v or higher on flooded-cell batteries or a specific gravity reading of 1.280 or higher—or 13.2v on absorbed glass mat (AGM) or gel batteries. No matter the battery type, follow with a load test. Use a 100W, 2 ohm wire-wound test resistor with its movable lug set in the middle. Jumper this lug and one of the outer ones while at the same time having a multimeter connected to read 12VDC or more, to the battery. At the end of a 15-second countdown, the battery, whatever type, should retain a minimum of 10.5v. If any of these tests fail, the battery should be re-charged and retested, and if it fails again, it needs to be replaced.

Part 2

Further reading:
Charging system types
Motorcycle charging system evolution
Permanent magnet charging systems
The CBX1000 charging system