BONUS ELECTRICAL PUB!
PUB has thoughts to share on the subject of magneto capacitors (aka condensers)
In recent issues of RC a number of correspondents have queried Neil Cairns’ article on fitting an ‘Easycap’ to his magneto, which apparently restored easy hot starting. Quite rightly they ask whether the internal capacitor (the more modern term for the old condenser) should be removed first? Quite right, it should – usually.
Readers will all know that the capacitor somehow magically suppresses sparking at the points, and enhances it at the plug. My simple explanation is that when the points open, the magneto primary current has to stop. But this stoppage of current and its associated electro-magnetic field, results in a high voltage – a few hundred volts, and will draw an arc at the points immediately they start to open. To prevent this, current is allowed to flow briefly into a capacitor, which rapidly charges up (we are talking microseconds here) and opposes the current, a sort of electrical shock absorber, and by then the points are open. Most windings are approaching 80:1, so that 200 volts in the primary winding is reflected as 16 kilovolts at the secondary.
Original capacitors in Bosch and BTH magnetos are usually made up of stacked mica and foil, which have proven very robust and resistant to damp, so that even old and neglected items generally work (the magnetos themselves can still suffer with ‘shellacitis’, when damp-impregnated shellac insulation goes soft and runny at temperature, then hardens and seizes the magneto upon cooling). Lucas capacitors, however, utilised (oiled) paper and foil strips, rolled into a flattened shape, and inserted into a metal case, closed with solder. These do deteriorate with old age and damp conditions – they survived better in their day as most machines were used, and warmed up, almost daily.
There are two common failure modes for the Lucas capacitors. Sometimes the connection to its ‘live’ end (the earth connection is usually via the metal casing) vanishes, possibly due to damp and corrosion where wire joins this foil. Typically this results in extremely difficult starting, misfiring, and excessive sparking and burning at the points – the very reason for which capacitors are fitted. If such a magneto has a cut-out terminal it can be jury-rigged by attaching an external capacitor from the terminal to earth (the PUB Vincent carries such a capacitor in its toolkit). For a failure of this type, it would be reasonable to fit an EasyCap to the points without physically removing the internal item (and the EasyCap can be fitted to a magneto without a cut-out facility).
Neil Cairns, however, complained that his bike would cold start fine, but not when hot. This is more typical of old and tired magnetos with old and tired capacitors. The connection remains sound, but damp and chemical changes cause a deterioration of the insulation layer. When disconnected from the winding this insulation should read completely off-scale on an electronic testmeter on its biggest range (typically 10 or 20 megohms). In practice it may not, and professionals will always replace.
However, as that is a big task, and if the winding is OK I will sometimes put a magneto back together if the reading is over 10 MΩ cold, and 1 MΩ hot. This reading is a measure of how much electricity the capacitor leaks away from the sparks, and anything lower will affect hot start performance even on soft old nails (for Gold Stars, etc, nothing less than perfect readings are acceptable because high CR requires higher voltage sparks).
Note that the resistance reading will not
be immediate, but will start low and rise over some seconds (during which the capacitor is charging up to match the battery in the meter). In fact that is a characteristic to look for in the separate capacitors used by coil ignition systems. Reverse the leads and you will even see a negative resistance, that then goes through zero and rises again. It is a sign of a good capacitor. If this slow ‘charging’ process is not seen then the capacitor may have a failure of the previous type, whatever the reading.
Note, however, that you must hold the insulated part of your probes, or all you will be measuring is your own fingers.
If the magneto has a ‘leaky’ capacitor, then simply fitting an EasyCap to the points is not a satisfactory solution, although perhaps it may sometimes provide a small improvement. However, the leak is still there, as is twice the capacitance value, all of which slows the rise-time and probably reduces peak voltage. Unfortunately, replacing a magneto capacitor is not an easy task, in fact far from it. Should your bike of choice use a K series Lucas magneto (K1F, K2F, KVF) with the triangular fitting, then you are lucky, because the option exists to simply cut and resolder internal connections. This ‘only’ requires dismantling the magneto (fraught with risk if removal of brushes, earth brush, and safety gap screws is neglected) and then putting it back together – preferably reshimmed for zero end float.
If, however, your magneto is of the N series beloved of AJS / Matchless singles, or any of the Magdyno series, then disconnection alone is not an option. On these magnetos the capacitor also incorporates the retaining nut for the points screw that itself is an electrical connection to the points. Complete removal of the original capacitor is required, but then a dummy has to be fabricated to retain the points and connect them to the armature winding. This task goes much further than simply removing the armature from the magneto body (leaving a keeper in place please).
Following removal, the drive end bearing has to be extracted, and then the slip-ring in order to access the through-bolts that hold the armature together, although the capacitor actually resides at the opposite end of the armature. After unsoldering the connection, removing the original condenser, replacing a dummy or gutted original, and resoldering all the connections, everything can be rebuilt.
Not an easy task, and not recommended for the inexperienced – I got my experience when there was no alternative, but secondhand knackered items were cheap and plentiful. Currently, since original Lucas capacitors are not available (and NOS probably not trustworthy), most rebuilders have found alternatives, which they usually epoxy in place of the originals. Sometimes their choices are not good (two capacitors with the same values and voltage rating may be as different as Manx and 16H engines), and a subsequent failure will mean another expensive rebuild. It is at this point that I recommend using the EasyCap, because it removes the component from deep inside the armature to on the points backplate.
The tiny item selected by Brightspark is a modern ceramic device, which does not exhibit the ‘self-healing’ quality claimed by the original paper-foil type, but has a habit of failing to a dead short (no sign of a spark). For this reason the type is not one I would recommend for embedding in the armature (nor would my tame magneto man, who has had to repair magnetos previously repaired thus), but mounted on the points it is no problem, as we can access them even at the roadside. Just order and carry a spare (maybe already fitted to a spare points block).