Motorcycle Spark Plugs
Perhaps the only part of a motorcycle that is taken for granted more than tires are the spark plugs. That is probably due to the dramatic improvement in ignition systems, fuels, fuel delivery systems and the plugs themselves over the past few decades.
They have gotten so reliable, they don’t often demand our attention — but it is still a good idea to know a few key things about them and give them a look from time to time — before they force us to.
In the 1970s, whether you rode a bike with a four-stroke or two-stroke engine, spark plugs introduced themselves into your riding life much more often than they do today — by fouling out or failing in some way, or just by having such short service lives. Of course, back then, if you rode a bike with two stroke engine, you just didn’t go anywhere without at least one spare plug – or more than one if you were smart.
These days, spark plugs benefit from the power of high energy electronic ignition systems, the precision of electronic fuel injection, higher quality, cleaner burning fuels and improved materials such as platinum, copper and iridium and design.
Still, the fundamentals are the same: the spark plug must deliver just the right amount of energy to ignite the fuel at exactly the right time. Seems easy enough until you consider the facts of the matter as explained by motorcycle engine technology master, Kevin Cameron in his book, “Sportbike Performance Handbook” :
“The ignition system quickly builds up a voltage difference across the spark plug electrodes. Any stray electrons in the gap are violently accelerated by this voltage difference, and they slam into atoms in their way, liberating more electrons. This is gap ionization, which makes the gas in the plug gap conductive. Almost instantly, a shower of electrons develops, which becomes a considerable electric current across the gap. Bombarded by these conduction electrons, whatever is in the gap is highly heated. This breaks up chemical compounds, allowing atoms from fuel and air to recombine in the energy-releasing process we call combustion.”
And you thought it was just an electrical spark setting some gas vapor on fire. So, this takes us to the top five things you need to know about your bike’s spark plugs.
1. The science behind how ignition happens:
It’s not just about making some high voltage electricity jump a gap; as Cameron explained above, it is about electrons violently slamming into atoms liberating more electrons in an instantaneous chain reaction of gap ionization, chemical destruction and reconstruction as combustion. Cameron explains that each cycle of fuel being drawn into the cylinder, compressed and ignited in an engine turning 10,000 RPM takes about one hundredth of a second!
2. Importance of the gap:
The spark plug has two components that create the gap — the center wire and the ground electrode. Generally speaking the center wire will have the negative (-) charge and the ground electrode the positive.
The larger the gap, the larger the amount of fuel/air mixture subject to gap ionization. If the gap is incorrect, too narrow or too wide, it can cause misfiring, pre-ignition, poor fuel economy, power loss and potentially even engine damage over time.
3. All spark plugs are not alike — different types of plug gap configuration exist:
In general, they are the standard plug gap where the center wire tip is roughly even with the edge of the threads and the ground electrode protrudes a beyond the level of the threads.
A projected tip plug has the insulator around the center wire and the center wire itself projecting beyond the level of the threads and the ground electrode correspondingly further out. Projecting the tip into the combustion chamber allows for more uniform flame spread on ignition.
A fine wire plug uses a smaller diameter center wire than a standard plug. A retracted gap plug has the center wire and the electrode set down inside the threaded portion of the plug, with the ground electrode attached to the inside of the threaded portion. The surface gap plug doesn’t use the standard ground electrode and center wire configuration. The difference between these plugs is much more than cosmetic—their performance characteristics are very different and they are designed for very specific engine, fuel and ignition system types.
For that reason, experimenting with changing out spark plugs must not be done willy-nilly. Using only the spark plug types specified by your bike’s manufacturer is the best way to optimize performance, prevent potential engine damage and protect your warranty.
4. Spark plugs come in different heat ranges:
Staying within the heat range designated for your engine is crucial. That said, a bike’s manufacturer may specify spark plug heat range options for different operating or environmental conditions. Be sure to check your owner’s manual and/or with the dealer before changing to a heat range other than what is specified.
While it is true that performance can be improved by changing spark plugs, this must be done within the manufacturer’s specifications. If you have engine modifications, consulting with an expert in performance modifications is a good idea when trying to maximize engine output without minimizing engine life.
A visual way to estimate heat range is by the length and girth of the insulator around the center wire. In general, a longer, slender insulator indicates a hotter heat range plug; shorter or thicker insulator construction indicates a colder heat range.
Manufacturers may offer more detailed information on the heat ranges of their products, as well as brand cross-reference charts to use in selecting spark plugs. Heat ranges are expressed in numerical terms with lower values (2-6) being hotter and higher values (7-11) being colder plugs.
5. Exotic center wire materials, designs and configurations are available:
But whether they deliver more reliable or higher performance or longer service life than a good, fresh set of the standard spark plugs specified for your machine depends on a lot of factors.
One factor that may affect your decision to give those types of plugs a try is cost—some types of plugs can be amazingly pricey.
Asking your motorcycle dealer or checking around among service professionals can provide some helpful information on which variations may work best for your bike. In my experience, even among spark plug options specified by the manufacturer, there are noticeable differences in service life.
For example in one of my newer bikes, platinum plugs simply outlast the standard plug by a wide margin. Other exotic precious metal materials are also available and may be worth checking out. Resistor plugs may be useful in some applications, as well.
Diagnosing engine anomalies can be done by taking a look at your spark plugs. Generally accepted normal condition for a used plug in a properly operating engine is a light ash or gray color on the ground electrode, center wire and end of the plug. Black carbon build-up may indicate too rich a fuel mixture or low voltage in the ignition (such as when cracked plug wire insulation is allowing voltage drop or misfiring).
Excessively wide gap and eroded center wire and/or ground electrode may indicate a worn out plug. Oily or wet plug surfaces may indicate severe engine wear at the oil control ring on the piston and/or valve guides. Evidence of melting of the center wire or ground electrode may indicate pre-ignition due to incorrect heat range or other problems.
When changing or cleaning/regapping your spark plugs, remember to check the plug connection caps and wires for cracks or other evidence of wear or damage that can lead to current loss or poor performance such as missing that may be incorrectly attributed to the spark plugs. New plugs on worn out connector caps or wires won’t solve the problem. Also, check, clean, re-gap or replace your spark plugs at the service intervals recommended by your bike’s manufacturer. Service intervals for spark plugs tend to be shorter for vintage bikes.
With all the improvements in spark plug technology, ignition systems, fuel systems and so on, is it still necessary to carry a spare spark plug and plug wrench? I say, why not? Even new, high tech spark plugs can just plain fail for one reason or another. For my old bikes, I tend to carry more than one spare plug in the saddle bag, but that’s just the old school in me. That spare spark plug will always cost less and take much less time than waiting for and paying for road service.
Your owner’s manual or various commercially available shop manuals may provide some excellent illustrations of the spark plug problems mentioned above as well as other types of problems. Here are a few online resources for more information:
The Green Spark Plug Company Fault Diagnosis chart:
- NGK Spark Plug Technical Diagnostic Information
- Champion Spark Plug Powersports Information
- Denso Spark Plugs Basic Information