The electrics of a classic motorcycle are simple, and they are also the source of many roadside failures. Most faults come down to a handful of causes: a poor connection, a weak battery, a dirty earth or a tired ignition component. With a cheap multimeter and some patience, you can find most of them.
6 volt and 12 volt
Older bikes are often wired for 6 volts, and later bikes for 12 volts. The system voltage decides the battery, the bulbs, the coils and the charging components. Never fit 12 volt parts to a 6 volt system or the other way round without understanding the whole circuit. A conversion from 6 to 12 volts is possible, but it needs new bulbs, a different regulator and often a different coil and horn, so keep the original parts if you convert.
Positive earth
On many British classics the positive pole of the battery is connected to the frame. This is called positive earth, and it is a common source of costly mistakes. Connecting a battery or a charger the wrong way round can destroy rectifiers, diodes and electronic parts. Before you connect anything, find out which pole is earthed, and mark the battery. Check the manual or ask an owners' club.
Charging
A generator or alternator produces current, and a regulator and rectifier control and convert it. Older bikes may have a dynamo, which makes direct current and has brushes that wear. Alternators are found on later models and produce alternating current that a rectifier converts.
Check the battery with a multimeter:
- 12 volt, engine off: a fully charged battery reads about 12.6 to 12.8 volts. Below 12.4 volts it is partly discharged.
- 12 volt, engine running above idle: the voltage at the battery should be higher, typically about 13.8 to 14.5 volts. If it does not rise, the charging system is not working. If it climbs well above 15 volts, the regulator is faulty.
- 6 volt systems: the figures are about half. Check the manual for the exact values.
Earths and connections
A classic frame is also part of the circuit. Paint, rust or grease between the earth lead and the frame can cause faults that look like a failed component. Clean and tighten every earth connection first. Check the connectors, the bullet connectors, the fuse holders and the switches. Pull gently on each wire and look for green corrosion at the ends.
To find a bad connection, measure the voltage drop. With the circuit switched on, a meter across a good connection should read close to zero. Anything above a few tenths of a volt is a poor connection.
Wiring
- Look at the age and condition of the harness. Brittle, cracked or sticky insulation is a fire risk.
- Look for household joints, tape and twist-on connectors. They are signs of earlier shortcuts.
- Most classics have few or no fuses. An inline fuse for the main feed is a cheap and safe upgrade that does not show.
- If the harness is poor, a new loom made for your model is a better solution than repairs.
Points ignition
Traditional ignition uses contact breaker points, a condenser (capacitor) and a coil. The points open and close as the engine turns, and they must be set correctly:
- The gap. Set it to the figure in the manual, often about 0.4 mm (0.015 in), and with the points fully open on the high point of the cam.
- The timing. The spark has to occur at the right moment. Set it statically with the marks, or with a strobe light if the bike has timing marks.
- The condition. Burnt or pitted points and a failed condenser cause weak spark and poor running. Replace them as a pair.
- Cleanliness. A trace of oil on the points will stop the bike.
Points need adjusting from time to time as the heel wears. This is normal for a classic.
Spark plugs
Use the type and heat range in the manual. A plug that is too hot can damage a piston, one that is too cold will foul. The colour of the insulator after a run tells you about the mixture: light brown is normal, black and sooty means rich, white means lean.
Electronic ignition conversions
Kits that replace the points with a sensor and an electronic module exist for many models. They give a stable timing that does not drift, easier starting and no regular adjustment. They also change a part of the bike, so keep the original components, and check whether a conversion is accepted for events or for a certification of originality, for example for the FIVA card. See the FIVA card.
Safety
- Disconnect the battery before you work on the wiring.
- Do not bypass a fuse with a wire.
- Fit a battery isolator if you store the bike, or remove the battery and keep it on a maintainer.
See also waking up a bike from storage.
