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Guide to Reading Spark Plug Performance

User Rating: / 115
Written by David Tise (Bulldog)   
Friday, 11 June 2004

David is a Technical Moderator, for Baron's Custom Accessories Technical Forum

The ability to read spark plugs can tell a mechanic a lot about the state of tune of an engine and about some of the problems an engine may be experiencing. In the absence of other more sophisticated equipment, the simple act of removing the spark plugs and examining them under a low power magnifying glass can help jet the carburetor, set ignition advance, recognize additives in fuel and many other conditions. While no one can learn to read spark plugs by simply studying a short article like this, you can learn the basics. 
What Does A Spark Plug Really Do?

The definition of the spark plug's purpose is straight forward. A spark plug seals the combustion chamber. It conducts a spark that is generated in the ignition coil into the combustion chamber and provides a gap for that spark to jump across. Finally, it conducts the heat it picks up in the combustion process to the cylinder head and into the cooling system. When a spark plug provides these three basic items, it is then capable of initiating the combustion process, There is a large amount of technology which goes into the design and manufacture of a spark plug that meets these requirements and provides long life and superior engine performance. It is important to remember that there is no design that will allow a spark plug to generate more voltage or hotter spark because the spark energy is generated in the coil. It is also important to remember that spark plugs do not produce heat. They pick up heat from the combustion chamber and move it to the cooling system. This basic definition of a spark plug will benefit you as the features and designs of spark plugs are described. It will also help you to better understand the value of reading spark plug firing end conditions.


Do I Need To Use A Resistor Plug?

Today's spark plugs use resistors to eliminate radio frequency interference (RFI) which can affect on-board computer or ignition systems, AM and FM radio reception, and TV broadcasts. Each time the spark jumps across a gap, an electromagnetic field is created that can interfere with radio signals. By placing a resistor in the spark plug, we can substantially reduce RFI. Furthermore, the use of non-resistor plugs with today's complex computer systems can result in driveability problems, loss of performance and can even cause the computer to store trouble codes.

Many motorcycles produced now have computer controlled ignition and/or fuel injection systems. RFI can effect these components. Several motorcycle aftermarket ignition modules are sensitive to RFI. Most notable among these are some modules produced by DynaTech. Use of proper spark plugs and ignition wires will eliminate the problems observed.


How Does Heat Range Affect A Plug?

As discussed, a spark plug is made up of ceramics, steels and nickel alloys. These materials do not have the ability to produce energy or heat. When we talk about the heat range of a spark plug, we're referring to its ability to move heat away from its tip or core nose into the cooling system. A cold spark plug would have a cooler tip temperature than a hot one. With today's fuels, we know that anytime the tip of the ceramic core nose goes below 850 F, carbon will build up and the spark plug will foul. We also know that if the tip temperature of the plug exceeds 1550', the metals will begin to break down. At approximately 1700, the plug will glow and can become a source of pre-ignition within the combustion chamber. Armed with this information, it becomes clear that maximum performance can be achieved with a spark plug that has a temperature of greater than 850 at idle, but no more than 1550' under wide open throttle.


Installation Tips

Here are a couple of tips to help assure the success of changing spark plugs. The first is to remove spark plugs from aluminum cylinder heads only when the engine is cool. Since aluminum and steel expand and contract at different rates, removing spark plugs from an aluminum cylinder head while hot can actually cause thread and cylinder head damage. Before installing the spark plugs, make sure the threads are clean and in good condition. If in doubt, run a thread chaser through the plug opening in the cylinder head. Most plug manufacturers recommend that you do not use an anti-seize compound, since one has already been applied to the plugs at the factory. Next, install the spark plugs finger tight and, it using a taper seat spark plug, use a ratchet to rotate 1/16 of a turn. If using a gasket seat type spark plug, after installing finger tight, turn 1/4 to 5/8 of a turn. If this procedure is followed, spark plugs will not back out nor will they seize in the cylinder head.


Reading the Plugs

We have provided a list of firing end conditions to help you identify situations that may occur in your vehicle. It is helpful to use a magnifying glass to view these deposits. We recommends the use of a 5 power magnifier with a light that will allow you to see small deposits on the corners of the spark plug and make an accurate diagnosis.


Normal Firing End

A grayish tan to while color indicates the correct heat range spark plug is in use, the fuel and ignition systems are in good shape and overall engine mechanical condition is good. Replace with new plugs of the same heat range. Refer to the owner's manual for recommended spark plug change interval.


Worn Out

Worn or rounded center and/or ground electrodes indicate excessive wear and can cause misfire during acceleration, hard starting or reduced fuel economy and damage to other secondary ignition components e.g., spark plug wires, coils and distributor cap with continuous use. When a plug has this type of appearance, it has simply exceeded its useful life. Replace with new spark plugs of the same heat range and design.



In cases of light detonation, small black or gray spots will be noticed on the core nose of the spark plug. In cases of severe detonation, insulators may be cracked and/or chipped. The same high pressure waves created during detonation can break spark plugs, damage intake valves and break pistons. Make sure that the correct octane fuel is being used, assure proper operation of emission and computer systems, and assure the correct heat range of spark plug is being used.


Mechanical Damage

Bent and/or broken electrodes and core nose indicate mechanical damage caused by foreign objects in the combustion chamber or improper reach spark plugs, Remove the foreign object from the engine and check the owner's manual for proper spark plug application.



Chalky white insulator with little or no coloration, accelerated electrode wear and possibly blistered or pitted electrodes are indications that a plug has overheated. The shell may also be discolored from light gray to a dark blue, almost black. Check for the correct heat range spark plug. Verify that ignition liming and air fuel mixtures are adequate. Engine overheating and restricted exhaust systems can also cause this condition.


Ash Deposits

Light brown deposits encrusted on the ground and/or center electrodes indicate ash deposits. This situation is caused by oil and/or fuel additives. When the deposits are found on only one side of the spark plug core nose, it is usually considered to be a problem with the cylinder head (valve stem seals or valve guides). When they are found on both sides of the spark plug, it is often considered to be a problem sealing at the piston rings. This condition can mask the spark and, in some cases, contribute to misfire. Check for worn valve guides and valve stem seals and/or piston rings. The spark plug is the correct heat range and was a victim of the engine's condition, not the cause of it. We does not recommend the use of fuel additives which leave deposits on the core nose of the spark plug.


Oil Fouled

Symptoms of oil fouling include black oily coating caused by poor oil control. This situation is more severe than what is seen with the ash-fouled spark plugs and usually represents advanced engine wear. When the oil enters the combustion chamber and covers the core nose of the spark plug, the spark no longer arcs across the gap. Rather it takes the easier path to ground by tracking down the oil on the core nose. This results in a complete cylinder misfire condition. Check for worn valve guides, valve stem seals and/or piston rings. Replacing the spark plug may help for a short time, but the new plug will soon foul.


Initial Pre-Ignition

Signs of the spark plug being hot or blistered and/or melted center and ground electrodes are indications of initial preignition. Check that the correct heat range spark plug is being used, assure ignition timing and air fuel mixture are appropriate, assure entire ignition system is functional and check its specifications. Routing of spark plug wires on some engines can contribute to cross induction which will lead to pre-ignition. Excessive carbon deposits in the combustion chamber may contribute as well.


Sustained Pre-Ignition

Melted center and/or ground electrodes and/or a melted insulator are symptoms of sustained pre-ignition. See initial preignition, description above. Also expect to find damage to the pistons and/or exhaust valves.


Splash Deposits

Small islands of contaminants on the insulator indicate splash deposits. Replace with new plugs of the correct heat range. The use of fuel additives, carburetor and choke cleaners or other aggressive solvents before installing new plugs is the most common cause of this condition.


Carbon Fouled

Soft, black, sooty, dry-looking deposits indicate a rich air fuel mixture, weak ignition or wrong heat range spark plug (too cold). These carbon-based deposits are conductive, much like oil fouling, and will allow the voltage coming out of the center electrode of the spark plug to track down the core nose rather than jumping the gap. This will result in an engine misfire and further aggravate the carbon fouled condition. Check for correct plug heat range. On fuel injected engines, check for sticking injectors, malfunctioning cold start valves and/or circuits. Also check for correct fuel pressure specifications. On computer controlled vehicles, the "limp home" computer mode will always result in a rich condition. Therefore, it is imperative that you check the operation and condition of the on-board computer system. On carbureted vehicles, check choke and choke pulloff, high float level, and needle and seat condition. On all engines, severe vacuum leaks can decrease manifold vacuum, resulting in a rich condition. Weak and/or damaged secondary ignition systems will fail to spark across the gap lowering combustion chamber temperatures and promoting carbon deposits. This condition could also result from continuous low speed driving or poor cylinder compression.


Fuel Additives

Red to purple deposits on one side of the core nose are an indication of a fuel additive. While many of these deposits are non-conductive and do not contribute to lack of performance, some fuel additives contain octane boosters that leave conductive deposits on the core nose. Care should be taken to select fuel additives which are compatible with ignition systems and do not contain conductive materials such as octane boosters.

Ride Safe & God Bless


Questions should be asked in our forum (Use discuss link below). The forum is very active and you stand a good chance of getting your questions answered there. If you would like to leave feedback for the author, or have additional information you think will benefit others, please use the comment section at the bottom of this page.

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DISCLAIMER: This information and procedure is provided as a courtesy and is for informational purposes only.  Neither the publishers nor the authors accept any responsibility for the accuracy, applicability, or suitability of this procedure.  You assume all risks associated with the use of this information.  NEITHER THE PUBLISHERs NOR THE AUTHORs SHALL IN ANY EVENT BE LIABLE FOR ANY DIRECT, INDIRECT, PUNITIVE, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, OF ANY NATURE ARISING OUT OF OR IN ANY WAY CONNECTED WITH THE USE OR MISUSE OF THIS INFORMATION OR LACK OF INFORMATION.  Any type of modification or service work on your motorcycle should always be performed by a professional mechanic. If performed incorrectly, this procedure may endanger the safety of you and others on your motorcycle and possibly invalidate your manufacturer’s warranty.

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  Comments (8)
Written by Newf1, on 01-31-2017 09:07
great info about plugs :x
spark plugs
Written by ASHOLENELS, on 07-29-2013 13:19
Can any one tell me the best plugs for my 2000 1600 road star, any help most welcome
Written by bigdescort, on 07-14-2013 11:20
what is cap setting on spark plug for 05 roadstar with jet kit
Plug reading
Written by efteland, on 03-20-2013 19:24
Do you have to look at all 4 spark plugs? Or could I just look at the outer ones that are easier to get to?
spark plugs
Written by 1LoneWolf, on 09-08-2011 11:53
great article Dave....great info for anyone that didn't know the importance of spark plugs :zzz
Spark plugs
Written by ghlaise, on 08-10-2010 06:22
Can any one tell me the best plugs for my 2000 1600 road star, any help most welcome :)
Great Job
Written by Starider, on 02-26-2010 15:32
any Plug and Wire recomendations?
Well Done
Written by BrooklynBiker, on 05-10-2009 05:17
Great Article, Thank you David.

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