Spark Plugs

November 17, 2009 by mike.cox

Most if not all bike ignitions are poor cousins of the car world.
Minimal energy to fire plugs was quite common on bikes from the year dot right up to the seventies bikes. Only then did CDI and other improved designs arrive.
Bikes need all the ignition help you can give them.
 
Apart from better HT coils and best quality ht leads, the sparkplug can offer increased spark energy if you know what to ask for.
One type of special plug is the one with a small centre electrode. If the centre electrode is thin, then it has to be made of special materials. And if special materials are used, the price is high.
 
These plugs have precious-metal electrodes. Conventional plugs have thick, blunt electrodes made of an alloy that's mostly iron, with a little nickel added to lend resistance to erosion.
 
Special-electrode plugs have a ground post made of ordinary nickel-iron alloy, but a centre electrode of something much more costly - which may be a silver alloy, or gold-palladium, or platinum, etc. Bosch still favours platinum; Champion, ND and NGK offer plugs with electrodes in materials ranging from silver to tungsten.
 
Platinum and gold-palladium alloys can survive the combustion chamber environment as very small wires, and in that rests their great advantage. Electrons leap away from the tip of a small-diameter, sharp-edged wire far more easily than from one that's fatter and rounded. So the fine-wire plug requires less voltage to form a spark than one with conventional electrodes, and the difference becomes increasingly biased in the former's favour as hours in service accumulate and erosion blunts the iron-alloy electrodes.
 
The standard plug with the normal round centre electrode and the flat blade type earth bar, once they have both become rounded off by erosion, you can bet the energy to fire the plug has increased by 40% or more.
 
There are, of course, drawbacks with precious-metal plugs: they are more expensive, ten times more is a fair guess, and they are very sensitive to excessive ignition advance. The overheating you get with too much spark advance effects the centre electrode before it can be detected elsewhere in an engine and when subjected to this kind of mistreatment fine-wire electrodes simply melt. In one sense this is a disadvantage, as it means the ruination of expensive spark plug. Seen in another way it's a bonus feature: it is better to melt a plug electrode than an engine.
 
A final variation on the basic spark plug you may not be aware of is something NGK calls a "booster gap," and is known at Champion as an "auxiliary gap." By any name it's an air gap built into a plug's core, and it improves resistance to fouling.
 
Conductive deposits on a plug's insulator nose tend to bleed off the spark coil's electrical potential as it is trying to build itself up to spark-level strength. In many cases the spark jumps down inside the body of the plug and not across the centre and the earth blade.
If so much energy is shunted in this way that firing does not occur, we say the plug is "fouled."
 
It is possible to clear a lightly fouled plug by holding the spark lead slightly away from the plug terminal and forcing the spark to jump across an air gap. The air gap works like a switch, keeping plug and coil disconnected until the ignition system's output voltage rises high enough and is backed by enough energy to fire the plug even though some of the energy is shunted by the fouling deposits.
 
Mechanics first discovered this trick; plug makers have incorporated it into some of the plugs they sell, and booster/auxiliary gap plugs work really well in bikes with an ignition system strong enough to cope with the added resistance. Such plugs more or less mimic the fast-voltage-rise characteristics of CDI systems - and offer no advantage used in conjunction with a capacitor-discharge ignition.
 
If you were to make yourself a plug gap, which were commercially available from the early seventies onwards and looked like a round quash ball flattened at both ends where the ht leads screwed in, all you need is a piece of clear fuel line about 30mm long.
Cut the ht lead somewhere along the lead.
Trim back both ends by 3mm or so.
Push the fuel line over one side of the ht lead. Then do the same on the other side.
Keep the two exposed copper wires apart by 3-5mm and that’s it.
Superglue the leads into the fuel line so they won’t pull apart.