Spark Plugs

[Fubar]

Probably the best explanation I've seen....

"Plug fouling causes can include:
Improper use, or abuse, of choke.
Continuous low engine speed.
Air/fuel mixture adjusted too rich.
Too cold spark plug heat range.
Lowered compression pressure along with the inherent engine oil entering the combustion chamber.
Over retarded ignition timing.
A general deterioration of the ignition system.
In this case let’s look at the plugs and, more importantly, the heat range. We’ll start, strangely enough, at the beginning…

What is the function of a spark plug?

The spark plug has two primary functions:
1) To ignite the air/fuel mixture. Electrical energy is transmitted through the spark plug, jumping the gap in the plugs firing end if the voltage supplied to the plug is high enough. This electrical spark ignites the air/fuel mixture in the combustion chamber.
2) To remove heat from the combustion chamber. Spark plugs cannot create heat; they can only remove heat. The temperature of the end of the plug’s firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. The “heat range” of a spark plug is defined as its ability to dissipate heat from the tip.

So what exactly is a spark plug “heat range”?

The heat range has nothing to do with the actual voltage transferred through the spark plug. Rather, the heat range is a measure of the spark plugs ability to remove heat from the combustion chamber. The insulator nose length and its ability to absorb and transfer combustion heat, the gas volume around the insulator nose, and the materials/construction of the center electrode and porcelain insulator, determine the heat range.

Given identical spark plug types, the difference from one heat range to the next is the ability to remove 150 to 200 degrees F from the combustion chamber. A long nose spark plug forces the heat from the tip to travel farther before it is absorbed by the cylinder head, which causes retention of more heat in the plug tip - making the plug “hotter” than a similar plug with a shorter nose. Engine temperature will affect a spark plugs operating temperature, but not the plugs heat range.

Heat range numbers go up as speed, load and temperature go up. In other words, higher numbers are colder plugs. A colder plug simply has a better ability to cool itself. When combustion chamber temperatures rise, the plug sees higher temperatures approaching its critical temperature of 1650 degrees F, over which the ceramic around the tip will blister and the electrodes will melt. Pre-ignition and detonation caused by an overheated tip is likely to cause severe engine damage. To avoid these plug failures some people must go to a colder plug.

The reason we cannot use a colder plug from the outset is fear of never getting the plug warm enough. If the tip temperature is below 850 F, the insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits. These deposits will accumulate and foul the plug gap, leading to a misfire. Plug tip temperature should remain between 850 and 1550 degrees F to reach its “self-cleaning temperature”. This is normally reached during high engine or vehicle speeds, and has no relationship to spark quality or intensity. This “self-cleaning temperature” allows the plug to burn off deposits accumulated at lower speed and idle conditions. If this temperature is not consistently reached, you will have definite fouling problems.

So, which heat range is right for MY engine?

There is no “correct” spark plug heat range for a specific engine. You should base your choice on many factors.
Compression ratio – a higher compression requires a colder plug.
Gearing - running taller, or smaller number, gears means also running a colder plug.
Weight - heavier bikes (or heavier riders and/or passengers) need a colder plug.
Wind resistance - the less aerodynamic, the colder the plug.
Quality of ring seal - poor oil control requires a hotter plug.
Power developed - increased power = increased heat = colder plug.
Fuel mixture - leaner = colder, richer = hotter plug.
Altitude - the higher up your travels, the higher up your heat range.
Throttle use - apply more throttle apply a colder plug.
Cruise speed - run faster, run colder.
Ambient air temperature – higher temperatures equate to a colder plug.

A couple of thoughts to leave you with – if you MUST clean plugs.
Never sandblast fouled plug until after the oily deposit has been removed with clean solvent and dried.
How do you know that you’ve managed to remove all the blasting debris from way down between the shell and center insulator? And where is any debris that you missed going to go?
Never use a wire brush to clean spark plugs. The bristles will leave electrically conductive “pencil marks” on the insulator nose.
It is widely held that plug cleaning will almost always result in less than satisfactory results.

I hope that this helps you some. Once you boil it down, it is pretty simple stuff. "

- Little Tommy
RIP

[Mominatrix]

Great read! Thanks Fubar! :mrgreen: