Classic components – Miller coils
Motorcycle ignition coils (aka spark coil) are electromagnetic induction coils. In common with those employed with many types of petrol engines, they transform the battery’s 6V or 12V into thousands of volts to produce a spark at the plug.
Ignition coils for classic motorcycles, cars, etc, comprise a laminated iron core concentrically wound with approximately 300 turns of thicker copper wire – the primary winding – and approximately 25,000 turns of thin copper wire, the secondary winding. An internal or external current limiting resister is often involved.
With the ignition system switched on when the contact breaker closes, after a fractional delay due to inductance of the coil, battery supplied current flows through the coil’s primary winding, producing a magnetic field in the iron core and air (or oil filled) space about the core. As the contact breaker (or electronic switch) opens, the current oscillates between the coil formed inductor and the capacitor (condenser) connected to the contact breaker, altering the magnetic field of the coil’s core inducing high voltage in the secondary windings, which transmits to the sparking plug via the high tension cable.
Design systems vary for electronic ignition, but a coil is still required, which may be independent to or incorporated into the system’s power box.
Modern coils
Traditionally the coil’s primary and secondary windings are applied concentrically to a straight laminated iron core. Many modern coils are considered more efficient as they employ a loop rather than a straight core.
Some motorcycles have double-ended coils.
Voltage
Commonly, motorcycle coils are available to suit 6V and 12V systems and should be used appropriately. To complicate the issue, some makers wired 2 x 6V coils in series for 12V systems and 3, 4.5 and 9 volt coils have been marketed. Maker’s wiring diagrams will assist.
How many?
With single cylinder machines that’s easy; usually one unless two are wired in series. Taking the example of a Triumph Trident, originally these had three contact breaker sets, three capacitors and three coils, so one of each making a set for each cylinder. However, other models such as Ariel Square Fours and some twins employed a distributor (like multi cylinder car engines) so usually only one each of the contact breaker, capacitor and coil is required.
Connections
Many coils both from period and aftermarket are marked with -ve and +ve, making wiring easy! Equally easy are Lucas and other coils marked SW (SWitch) and CB (Contact Breaker) or S and C, but some coils are numbered; again, refer to the maker’s wiring diagram. Many coils will work – but less efficiently – if connected the wrong way.
Replacement and Damage
Some enthusiasts state ignition coils never fail; actually, they do. Failure may be due to long-term use, engine heat, poor storage, abuse including leaving the ignition switched on for long periods with engine stationary causing the coil to get hot, and other reasons.
Trembler coils
From the launch of the George Bouton designed ‘make and break’ (contact breaker), early motorcycle, car and other engines with battery powered coil ignition systems employed trembler coils until after the appearance of designs based on the 1908 patented DELCO (Dayton Engineering Laboratories Company) coil, which was like the ignition coils used today. Once the DELCO type coil gained popularity, trembler coils largely became obsolete, although they remained a fitment for some stationary/ industrial petrol engines and various cars (including the Model T Ford) for some years.
The trembler comprises primary and secondary windings about an iron core. Electrical power is provided by a battery and engine driven contact breaker set (with cam etc) times the ignition, as with later designs. The created high tension current then passes to a second adjustable contact breaker sited to the trembler coil, which vibrates on command of passage of high tension current, resulting in the release of a brief spray of sparks at the sparking plug. Most trembler coils emit a buzzing sound each time a spray of sparks is issued.