Sparklers: Spark plugs come in different
sizes and heat ranges.
Since the advent of four-strokes,
no one talks about spark plugs
anymore. I remember back in the
two-stroke days when I wouldn’t
go to the races without two or three
spare plugs. How often should I
replace the plug in my 2012 Honda
Your spark plug acts as a
grounding conductor for your bike’s
ignition. Electrical current generated
by the magneto is sent to the coil,
where it is stepped up in voltage and
relayed to the black box. When the
piston reaches a calculated position
before top dead center, the energy
is routed to the spark plug. As the
current reaches the plug’s center
electrode, it jumps the gap, where
it ignites the engine’s compressed
fuel/air mixture. The average 250cc
machine will ignite the compressed
fuel 150 times per second or more.
Why do spark plugs fail? A spark
plug can fail for a lot of reasons.
Here is MXA’s hit list.
Fuel: Gasoline, like bath water, is
a conductor of electricity. Electricity
always takes the path of least
resistance. If your engine is loaded
up (too much fuel in the combustion
chamber), the spark will automatically be conducted by the gasoline
down the plug’s porcelain to the
grounded plug shell. In short, the
spark won’t jump the gap—it will go
around the gap.
Oil: If your fuel/oil is poorly mixed
or of a bad grade, the spark plug’s
flame kernel will not completely
burn the oily residue off the plug.
Wet oil will coat the porcelain and,
in short order, the built-up oil film
will provide a conductor of less
resistance between the electrode
and the plug shell.
Carbon deposits: Carbon
deposits of unburned oil residue can
build up around the porcelain
insulator. They prevent the plug
from operating at full temperature,
which, in turn, doesn’t allow the
spark to burn the residue off the
plug. Since the built-up carbon will
conduct electricity with less resis-
tance than the plug gap, the current
will be diverted through the carbon
to the electrical ground without
creating a spark across the gap.
Gap erosion: The spark plug
jumps the gap from the sharpest edge of the plug’s electrode.
Unfortunately, every time the plug
fires, it eats away at the electrode
and ground strap. When the sharp
edge is worn down, it becomes
harder for the spark to jump the
gap. You should never run a plug
long enough for gap erosion to
become a performance problem.
Internal damage: Too much
compression, bad gas, cheap oil
or lean jetting can create a lean
condition in the engine. When the
combustion chamber becomes
extremely hot, the fuel can be
ignited too early by hot spots
created by carbon deposits. This
pre-ignition condition, commonly
referred to as “detonation,” can
damage the spark plug.
External damage: Rough
handling or over-tightening can
crack the plug’s porcelain and
cause the ignition to misfire.
Spark plugs on a two-stroke are
most vulnerable to oil contamination and carbon buildup, as excess
premix oil is turned into charred
carbon. Four-strokes don’t have this
problem, but their plugs still wear
out—mostly from gap erosion. Every
time the plug fires, it eats away at
the electrode and ground strap.
When the sharp edge is worn down,
the spark moves to the next closest
sharp edge, which is, of course,
farther away. As that edge gets
eaten away, the spark has to jump
to a new location. The erosion of
the sharp edges is hard to see, but
it exists. The center electrode will
round before the ground strap. If
your bike’s fuel/air mixture, timing
and carburetion are correct, your
spark plug should last until the
electrode’s edges are rounded off.
We would change to a new plug
every six months.
The arrival of the four-stroke
movement lessened the critical
importance of spark plugs at the
owner level—and most likely put
a serious bite in the bottom line of
the motorcycle spark-plug business.
Two-strokes, by their very nature,
were spark-plug dependent. If
there was any kind of issue with a
two-stroke, step one was to check
the plug for spark, then clean the
plug or replace the plug.
Four-strokes still need to have
new plugs at regular intervals, but
most riders don’t change their plugs
until something goes drastically
Here is MXA’s guide to living with
(1) Set the gap. Setting the gap
has become a lost art. Most modern
riders just open the package and
throw the new plug in the bike. Not
a good idea. As a rule of thumb,
you should check the gap of a new
spark plug before installation and
every time the plug is out of the
cylinder head. The gap should
always be adjusted to the factory’s
standard setting. It is not uncommon
for hop-up tuners to set the gap a
little tighter when they raise the
compression. Why? The smaller the
gap, the less voltage required to
jump the gap.
( 2) Check the edges. The spark
prefers to jump from the sharpest
edge of the electrode to the
sharpest edge of the ground strap.
With each firing, the spark eats
away at the metal that it jumps to
and from. Plugs round off after six
months of regular use.
( 3) Check the heat range. A
plug’s heat range only dictates how
quickly the plug resists the transfer
of heat to the cylinder head, not
the intensity of the spark. A higher
heat range means the plug will
cool slowly, causing it to stay hotter
during operation. A cooler plug
dissipates heat faster and sparks at
a cooler temperature during
operation. Plugs with a “ 7” heat
range are hotter than “ 8” plugs,
which are hotter than those with
a “ 9” rating. If your bike uses a “ 9”
plug, a one-step-cooler plug would
be a plug with a “ 10” heat rating.
( 4) Install it gently. Too much
torque can damage the plug’s
internal seals or break it at the
gasket, especially on the smaller-diameter spark plugs. Tighten plugs
with 10mm threads to 7 to 11 foot-pounds, 12mm plugs to 11 to 19
foot-pounds and 14mm plugs to 15
to 22 foot-pounds.
( 5) Clean or replace? The
general rule is that once a plug
fouls, it is history. At the track, it’s
almost impossible to remove all of
the carbon residue from whatever
caused the plug to foul in the first
place. That is why a fouled plug
that is simply sprayed off with
contact cleaner and reinstalled is
more susceptible to fouling again.
Many backyard mechanics
sandblast their plugs, but most
plugs that need to be sandblasted
are past their service window