What's up, Sparky?

One of the essential component of any engine
is the spark plug.

'what is a spark plug?'

Here is it from Wikipedia:
A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed aerosol gasoline by means of an electric spark. Spark plugs have an insulated center electrode which is connected by a heavily insulated wire to an ignition coil or magneto circuit on the outside, forming, with a grounded terminal on the base of the plug, a spark gap inside the cylinder.

pic from http://www.centuryperformance.com

Thus the primary functions of the spark plug is:

a. To ignite the Air/Fuel Mixture
b. To remove the heat out of the combustion chamber

Also from http://www.centuryperformance.com, it has
a good article on Influences On Spark Plug Temperature
and Performance which I think it can be applied to karting.

Because some of us may kart in different parts of the world
with very different climate, so we will have to be concious
about using the correct spark plug for the engine.

Here you go with the detailed yet easy to understand article:

Below is a list of possible external influences on a spark plug's operating temperatures. The following symptoms or conditions may have an affect on the actual temperature of the spark plug. The spark plug cannot create these conditions, but it must be able to deal with all the levels of heat, otherwise performance will suffer and engine damage can occur:

1) Air/Fuel Mixtures seriously affect engine performance and spark plug temperatures.

• Rich air/fuel mixtures cause the tip temperatures to drop, causing fouling and poor drivability
• Lean air/fuel mixtures cause plug tip and cylinder temperatures to increase, resulting in pre-ignition, detonation, and increase the possibility of spark plug and internal engine damage.
• It is important to read spark plugs many times during the tuning process to achieve the optimum air/fuel mixture.
• Computer-controlled engine applications do a pretty good job of maintaining the optimum air/fuel mixture by using the various sensors that report back to the ECM, but even during periodic maintenance the spark plugs can offer insight as to how the engine is performing.

2) Higher Compression Ratios, Forced Induction, and Nitrous Oxide will elevate spark plug tip and in-cylinder temperatures.

• Compression pressure, defined as cylinder pressure (not to be confused with static compression ratio, though it is also part of this calculation), can be increased be performing any of the following modifications.

• Reducing combustion chamber volume (higher compression domed pistons, smaller chamber cylinder heads, milling the heads, lowering the block deck height, thinner cylinder head gaskets), which are all examples of changing the static compression ratio.
• Adding a forced induction system to the engine (supercharger, turbocharger, nitrous)
• Camshaft change, where the changes in new cam timing affect the cylinder pressure in the engine.

• As compression pressure (cylinder pressure) increases, a colder heat range spark plug is required, as well as higher octane fuel and paying careful attention to ignition timing and air/fuel ratios.
  

3) Advanced Ignition Timing, or moving the point of ignition farther ahead of where the piston reaches TDC (top dead center).

• By advancing the timing 10° you can easily add 70°c to 100°c (158°f to 212°f) to the spark plug temperature.

4) Engine Speed and Load changes will increase the firing end temperatures of the spark plug.

• When a vehicle is cruising down the highway at a steady rate of speed the spark plug firing end temperatures will be fairly constant, affected by the load on the engine (weight of vehicle, aerodynamics, rolling resistance, type of road you are on, etc.). Changes in engine load will affect the tip temperature of the spark plug.
• If your vehicle is constantly running at a high RPM, carrying/pushing/pulling heavy loads, climbing mountain roads ... a colder plug may be required. You must read the plugs to see how they are handling this use.
• The heavier your vehicle, or greater the amount of work the engine sees (racing applications, heavy-loaded construction trucks, vans, RVs & motor homes), the more critical this becomes. Hopefully the engineers that built your vehicle did their homework, and you are using your vehicle within its intended design, that way the specified plug heat ranges are accurate. If you have deviated much from the intended use of your particular vehicle, plug reading becomes more important.

5) Ambient Air Temperature, defined as the incoming air that enters your engine through the air filter and into the combustion process.

• As air temperature falls, air density volume increases, resulting in leaner air/fuel mixtures. This creates higher cylinder pressures and temperatures which causes an increase in the spark plug's tip temperature. Fuel delivery should be increased in this scenario.
• As air temperature increases, air density increases, as does intake volume, and fuel delivery should be decreased.
• On computer-controlled applications, this is compensated automatically as the sensors report information to the ECM to make adjustments in the fuel curve. But, on carbureted applications you may need to compensate the jetting or available adjustments if you operate the vehicle primarily in constant extreme high or low temperatures.

6) Humidity, or moisture in the air.

• As humidity increases, air volume decreases. The result is lower combustion pressures and temperatures, causing a decrease in the spark plug's temperature and a reduction in available power.
• Along with increases in humidity, the air/fuel mixture should be leaner, but it will also depend on ambient air temperatures. The opposite changes are needed as humidity decreases.

7) Barometric Pressure and Altitude

• Affects the spark plug's temperature
• The higher the altitude, the lower the cylinder pressure becomes. As cylinder temperature decreases, so does the tip temperature.
• Many tuners will attempt to "chase after" optimum tuning by changing spark plug heat ranges.
• The real answer is to play with the jetting or air/fuel mixtures in an effort to put more air back into the engine.