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Old 12-17-2006, 11:55 AM
2kHondaCR500R's Avatar
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Default Re: How do you stop a plug fouling


The only time I have ever fouled a plug was for one of two reasons: 1) too rich on the carb jetting, or 2) too cold of a heat range plug.

If you know for sure your jetting is right (not rich or lean - spot on), try a step hotter plug. If it gets any detonation, then go back up to the stock heat range.

FWIW, I run a NGK #9 heat range plug in my CR 500, but with high compression and race fuel. When I run pump gas and lower compression, I use a NGK #8 heat range plug.

Check your ignition to make sure there isn't a voltage problem, or connection problem.

NGK Sparkplug Tech:

Tech Info - Spark Plugs Overview

Spark plugs are one of the most misunderstood components
of an engine. Numerous questions have surfaced over the years, leaving many people confused.

This guide is designed to assist the technician, hobbyist, or race mechanics in understanding, using, and troubleshooting spark plugs. The information contained in this guide applies to
all types of internal combustion engines.

Spark plugs are the "window" into the engine , and can be used as a valuable diagnostic tool. Like a patient's thermometer, the spark plug displays symptoms and conditions of the engine. The experienced tuner can analyze these symptoms to track down the root cause of many problems, or determine air/fuel ratios.


The spark plug has two primary functions:

Ignite air/fuel mixture
Transfer heat from the combustion chamber
Spark plugs carry electrical energy and turn fuel into working energy. A sufficient amount of voltage must be supplied by the ignition system to spark across the spark plug's gap. This is
called "Electrical Performance."

The temperature of the spark plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called "Thermal Performance", and is
determined by the heat range selected.

It's important to remember spark plugs do not create heat, they only remove heat. The spark plug works as a heat exchanger
by pulling unwanted thermal energy away from the combustion chamber, and transferring the heat to the engine's cooling
system. 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 porcelain insulator

A spark plug's heat range has no relationship to the actual voltage transferred through the spark plug. Rather, the heat range is a measure of the spark plug's ability to remove heat from the combustion chamber. The heat range measurement is determined by several factors; the length of the ceramic center insulator nose and its' ability to absorb and transfer combustion heat, the material composition of the insulator and center electrode material.

Heat rating and heat flow path of NGK Spark Plugs

The insulator nose length is the distance from the firing tip of the insulator to the point where insulator meets the metal shell. Since the insulator tip is the hottest part of the spark plug, the tip temperature is a primary factor in pre-ignition and fouling. Whether the spark plugs are fitted in a lawnmower, boat, or a race car, the spark plug tip temperature must remain between 500C-850°C. If the tip temperature is lower than 500°C, the insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits. These accumulated deposits can result in spark plug fouling leading to misfire. If the tip temperature is higher than 850°C the spark plug will overheat which may cause the ceramic around the center electrode to blister and the electrodes to melt. This may lead to pre-ignition/detonation and expensive engine damage. In identical spark plug types, the difference from one heat range to the next is the ability to remove approximately 70°C to 100°C from the combustion chamber. A projected style spark plug firing tip temperature is increased by 10°C to 20°C.

Tip Temperature and Firing End Appearance

The firing end appearance also depends on the spark plugs tip temperature. There are three basic diagnostic criteria for spark plugs: good, fouled and overheated. The borderline between the fouling and optimum operating regions (500&def;C) is called the spark plug self-cleaning temperature. The temperature at this point is where the accumulated carbon and combustion deposits are burned off.

Keep in mind the insulator nose length is a determining factor in the heat range of a spark plug, the longer the insulator nose, the less heat is absorbed, and the further the heat must travel into the cylinder head water jackets. This means the plug has a higher internal temperature, and is said to be a hot plug. A hot spark plug maintains a higher internal operating temperature to burn off oil and carbon deposits, and has no relationship to spark quality or intensity.

Conversely, a cold spark plug has a shorter insulator nose and absorbs more combustion chamber heat. This heat travels a shorter distance, and allows the plug to operate at a lower internal temperature. A colder heat range is necessary when the engine is modified for performance, subjected to heavy loads, or is run at a high rpm for a significant period of time. Colder spark plugs remove heat quicker, reducing the chance of pre-ignition/detonation. Failure to use a cooler heat range in a modified application can lead to spark plug failure and severe engine damage.

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

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

Rich air/fuel mixtures cause tip temperature to drop, causing fouling and poor driveability
Lean air/fuel mixtures cause plug tip and cylinder temperature to increase, resulting in pre-ignition, detonation, and possibly serious spark plug and engine damage
It is important to read spark plugs many times during the tuning process to achieve the optimum air/ fuel mixture
Higher Compression Ratios/Forced Induction will elevate spark plug tip and in-cylinder temperatures

Compression can be increased by performing any one of the following modifications:

a) reducing combustion chamber volume (i.e.: domed pistons, smaller chamber heads, mill ing heads, etc.)

b) adding forced induction (Nitrous, Turbocharging or Supercharging)

c) camshaft change

As compression increases, a colder heat range plug, higher fuel octane, and careful attention to ignition timing and air/fuel ratios are necessary. Failure to select a colder spark plug can lead to spark plug/engine damage

Advancing Ignition Timing

Advancing ignition timing by 10° causes tip temperature to increase by approx. 70°-100° C
Engine Speed and Load

Increases in firing-end temperature are proportional to engine speed and load. When traveling at a consistent high rate of speed, or carrying/pushing very heavy loads, a colder heat range spark plug should be installed

Ambient Air Temperature

As air temperature falls, air density/air volume becomes greater, resulting in leaner air/fuel mixtures.
This creates higher cylinder pressures/temperatures and causes an increase in the spark plug's tip temperature. So, fuel delivery should be increased.
As temperature increases, air density decreases, as does intake volume, fuel delivery should be decreased


As humidity increases, air intake volume decreases
Result is lower combustion pressures and temperatures, causing a decrease in the spark plug's temperature and a reduction in available power.
Air/fuel mixture should be leaner, depending upon ambient temperature.

Barometric Pressure/Altitude

Also affects the spark plug's tip temperature
The higher the altitude, the lower cylinder pressure becomes. As the cylinder temperature decreases, so does the plugs tip temperature
Many mechanics attempt to "chase" tuning by changing spark plug heat ranges
The real answer is to adjust air/fuel mixtures by rejetting in an effort to put more air back into the engine

Types of Abnormal Combustion


Defined as: ignition of the air/fuel mixture before the pre-set ignition timing mark
Caused by hot spots in the combustion chamber...can be caused
(or amplified) by over advanced timing, too hot a spark plug, low octane fuel, lean air/fuel mixture, too high compression, or insufficient engine cooling
A change to a higher octane fuel, a colder plug, richer fuel mixture,
or lower compression may be in order
You may also need to retard ignition timing, and check vehicle's cooling system
Pre-ignition usually leads to detonation; pre-ignition an detonation are two separate events


The spark plug's worst enemy! (Besides fouling)
Can break insulators or break off ground electrodes
Pre-ignition most often leads to detonation
Plug tip temperatures can spike to over 3000°F during the combustion process (in a racing engine)
Most frequently caused by hot spots in the combustion chamber.
Hot spots will allow the air/fuel mixture to pre-ignite. As the piston is being forced upward by mechanical action of the connecting rod, the pre-ignited explosion will try to force the piston downward. If the piston can't go up (because of the force of the premature explosion) and it can't go down (because of the upward mo-tion of the connecting rod), the piston will rattle from side to side. The resulting shock wave causes an audible pinging sound. This is detonation.
Most of the damage than an engine sustains when "detonating" is from excessive heat
The spark plug is damaged by both the elevated temperatures and the accompanying shock wave, or concussion


A spark plug is said to have misfired when enough voltage has not been delivered to light off all fuel present in the combustion chamber at the proper moment of the power stroke (a few degrees before top dead center)
A spark plug can deliver a weak spark (or no spark at all) for a variety of reasons...defective coil, too much compression with incorrect
plug gap, dry fouled or wet fouled spark plugs, insufficient ignition timing, etc.
Slight misfires can cause a loss of performance for obvious reasons (if fuel is not lit, no energy is be-ing created)
Severe misfires will cause poor fuel economy, poor driveability, and can lead to engine damage


Will occur when spark plug tip temperature is insufficient to burn off carbon, fuel, oil or other deposits
Will cause spark to leach to metal spark across plug gap will cause a misfire
Wet-fouled spark plugs must be changed...spark plugs will not fire
Dry-fouled spark plugs can sometimes be cleaned by bringing engine up to operating temperature
Before changing fouled spark plugs, be sure to eliminate root
cause of fouling

Last edited by 2kHondaCR500R; 12-17-2006 at 12:13 PM.
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Old 12-26-2006, 05:32 PM
Rockey's Avatar
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Default Re: How do you stop a plug fouling

Check the resistance in the coil if leaner carburation doesn't do the trick. You'd have to be really rich to cause that rapid of plug fouling.
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Old 01-03-2007, 05:28 PM
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Default Re: How do you stop a plug fouling

I have a feeling that after you clean the air filter the bike will run longer than 30 minutes on a fresh plug. One thing that I have found out after years of working on 2 strokes is that usually if they are going to foul a plug, it is when the bike is cold, not after 30 minutes of riding. Unless it is stop and go riding and you are running the gas too rich. Get rid of the Castrol quick stop 2 cycle oil and get some good synthetic stuff. You should be able to run 40:1 with quality oil and I would almost bet that your bike runs fine after that. If not, somebody probably jetted the bike too rich for your climate or you have leaky crank seals.
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Old 01-03-2007, 05:45 PM
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Default Re: How do you stop a plug fouling

i use to foul lots of plugs in my cr125 from babying it so i went to a br8eg from a br9eg and just started running it hard and i dont foul them any more.
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Old 01-03-2007, 07:14 PM
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Default Re: How do you stop a plug fouling

One thing that I dont know if anybody brought up is that if your bike needs a top end rebuild a 125 or 2 stroke in general has a tendency to foul plugs more often. You might want to check your compression, if it is under 120-130 you may be in need of a piston and rings.
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Old 01-03-2007, 07:22 PM
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Usflag Re: How do you stop a plug fouling

This one is in the KB now, nice work all.

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Old 01-04-2007, 07:14 PM
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Default Re: How do you stop a plug fouling

Originally Posted by Florida 393 View Post
This one is in the KB now, nice work all.

What exactly does "this one is in the KB now" mean?
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Old 02-13-2007, 07:26 PM
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Default Re: How do you stop a plug fouling

Hi, what plug are you running. I know standard is 10 my mate ran a 9 for ages and ages, kept telling me was always fouling plugs. told him time and time again to run an 8 plug, he didnt listen for ages, when he finally did he never fouled a plug again. I have always used an 8 in all my bikes for ten years with never a problem.
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Old 02-17-2007, 11:47 PM
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Default Re: How do you stop a plug fouling

I was running a 9, but then on my last ride I tried an 8, it still fouled and wouldn't start after 20-30 mins.
Could it be my stater coil??? My ignition coil tests ok, and my leads, but it seems funny it fouls after 30 mins, as though its loosing its spark.
I just bought a number 45 pilot jet and droped the needle 1 clip, and Im going to change to Motul 700 oil - see how it goes.
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Old 02-18-2007, 08:20 AM
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Usflag Re: How do you stop a plug fouling

Originally Posted by TUK101 View Post
What exactly does "this one is in the KB now" mean?
It means I have moved the contents of the thread into the Knowledge base forums so it is in the technical sections that can be readily accessed by those looking for answers to problems.

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