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Spark Plugs
(click for explanation of models)

Have you and your baby lost that spark?

 . . . sometimes it isn't that difficult to light up your life bike.

After a review of sources that deal with spark plugs one thing becomes very apparent - people often think that spark plugs are a complex subject. They don't have to be.

The heat range of a plug is probably the most misunderstood aspect of a spark plug.
As NGK states "The temperature of the spark plug's firing end must be kept low enough to
prevent pre-ignition, but high enough to prevent fouling."

Higher compression is used in the search for more power - but along with that power comes more heat. This means a spark plug of a colder heat range must be used, possibly with a smaller gap, to ensure correct ignition of the denser mixture of air and fuel. 'Correct ignition' here means the prevention of pre-ignition ('pinking').

Low compression engines need a hotter plug to prevent fouling and make sure that the process of ignition actually happens.

The heat range is defined as a plug's ability to dissipate heat. The rate of heat transfer is determined by:
- The insulator nose length
- Gas volume around the insulator nose
- The materials/construction of the center electrode and insulator
A spark plug's heat range has no relationship to the actual voltage transferred though the spark plug.

The N3 was standard fitment for both the BSA and Triumph Triples. Champion now offers the N3C - the "C" indicates a copper core for more accurate control of the heat range. In the chart below the right-hand list (with the "R" prefix, eg: RN3C) list indicates the resistor-type spark plug.

Assuming that everything else about the bike is mechanically in order (!) you should use a colder plug if your bike suffers from chronic pinking, a hotter plug if your bike suffers from fouled plugs.

A standard analysis chart of common spark plug problems (courtesy of Champion):

Firing End Analysis
NORMAL OIL FOULED CARBON FOULED
A normal condition is shown below. This plug has been running at the correct temperature in a "healthy" engine. Operating in such a desirable environment results in deposits that will be light tan or gray in color with most grades of commercial gasoline.
If LP Gas or natural gas has been used, the color will be almost brown.
A spark plug shorted by excessive oil entering the combustion chamber is shown below. This is often caused by piston rings or cylinder walls that are badly worn. Oil may also be pulled into the chamber because of excessive clearance in the valve stem guides, or badly worn valve stem seals. If the PCV valve is plugged or inoperative, it can cause a buildup of crankcase pressure. This condition can force oil and oil vapors past the rings and valve guides into the combustion chamber. Basically, soft, sooty carbon deposits, as shown below, have a dry, black appearance. If only one or two plugs in a set are fouled, it is a good practice to check for sticking valves, a cracked distributor cap, or bad secondary ignition wires. Fouling of the entire set might result from an incorrect heat range spark plug or an over-rich air/fuel mixture caused by a clogged air cleaner filter element, a sticking heat riser valve, or a faulty choke. Fuel injectors that malfunction can also lead to this condition.

Other causes include weak ignition system voltage or an inoperative pre-heating system (carburetor intake air) or poor cylinder compression.


WORN INSULATOR GLAZING MECHANICAL DAMAGE
This spark plug shown below has served its useful life and should be replaced. Voltage required to fire the plug has approximately doubled and will continue to increase the longer the engine operates.

Even higher voltage requirements (as much as 100%) above normal may occur when the engine is accelerated quickly. Poor engine performance and a loss of fuel economy are traits of worn spark plugs.

This condition may cause misfiring at high engine RPM. Shiny deposits usually suggest that temperatures have suddenly increased during hard acceleration. As a result, normal metallic deposits do not have a chance to slough off the plug and they melt and form a conductive coating which causes the misfire. Yellow or tan deposits, as shown below, usually indicate the use of leaded fuel. Mechanical damage to the firing end, as shown below, is caused by some foreign object in the combustion chamber. Since small objects can travel from one cylinder to another (because of valve overlap), the other cylinders should always be checked to prevent reoccurance of damage.

When working on an engine, it is advisable to keep the carburetor throat (or throttle body openings on central type fuel injected engines) covered. That precaution also applies to spark plug holes.


OVERHEATED SPLASHED FOULED ASH FOULED
An example of overheating is shown below. Note the dead white or gray insulator nose which appears "blistered." Electrode gap wear rate will be considerably in excess of that normally expected. This is often caused by overadvanced ignition timing, poor engine cooling system efficiency (scale, stoppages, low level), a very lean A/F mixture, a leaking intake manifold, or the use of a spark plug too hot for the application. Splashed fouling, as shown below, may sometimes occur after a long-delayed tune-up. Here, deposits accumulated after long periods of misfiring or low power operation may be suddenly loosened when normal combustion temperatures are restored after a set of new plugs has been installed. During a high-speed run, these materials shedding off the piston are thrown against the hot insulator surfaces. As shown below, a buildup of combustion deposits stems primarily from the burning of oil and/or fuel addatives during normal combustion.

These are normally nonconductive. However, when heavier deposits are allowed to accumulate over long mileage periods, they can "mask" the spark, resulting in a plug misfire condition.


GAP BRIDGING PREIGNITION DETONATION
An example of gap bridging is shown below. It rarely occurs in automotive engines. Gap bridging is caused by conditions similar to those that produce splash fouling. Combustion deposits thrown loose may lodge between the electrodes, causing a dead short and misfire. Fluffy materials that accumulate on the side electrode may melt to bridge the gap when the engine is suddenly put under a heavy load. As shown below, this condition produces melting of the center electrode, and somewhat later, the ground electrode and insulator. Usually one or a combination of several engine operating conditions are the prime causes of pre-ignition. It may originate from glowing combustion chamber deposits, hot spots in the combustion chamber due to poor control of engine heat, cross-firing (electrical induction between spark plug wires), or the plug heat range is too high for the engine or its operating conditions. As shown below, this form of abnormal combustion has fractured the insulator nose of the spark plug. Explosions that occur when the operating condition exists apply extreme pressure on internal engine components. Major causes include a faulty EGR valve, lean air/fuel mixtures, ignition timing advanced too far, and insufficient octane rating of the gasoline.

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