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No
matter what kind of bike, truck, buggy,
ATV, watercraft or snowmobile you operate,
chances are just about 100% that you
have one or more spark plugs firing the
fuel and creating the power required
for forward motion.
If
the plug(s) quits working, your engine
stops. Or maybe things start sputtering. Could
be the plug is beginning to foul.
So
you yank it out and take a look at it. But
what are you really looking for? What kind
clues can a visual check tell you about
what’s going on inside that engine?
Let’s
walk through it.
A
spark plug is inserted into the engine,
the engine is operated for a while, then
the spark plug is removed for examination.
It shows how hot the plug has been while
the engine was running and also tells
you about a variety of other symptoms. This procedure
is called “reading” the plugs, and it’s
probably the single most important guide
to tuning an engine.
Plugs
do a simple task: they make sparks to ignite
the fuel-air mixture in the combustion chamber. They
have to cope with heat, pressure, vibration,
noxious fumes, liquids and chemicals. When
you remove the plug, it can tell you how
bad (or good) things were in the combustion
chamber, just by its appearance.
WHAT’S
IN A PLUG?
The
bottom part of a plug is metal, threaded
to screw into a hole in the cylinder
head, and is often called the shell .
The diameter of the threaded section,
or the diameter of the hole it fits, is
specified in millimeters (mm). Common plug
diameters are 10mm, 12mm and 14mm. There
are many stories around about boneheads
trying to stuff a 14 mm plug into a 12 mm
hole.
The
length of the threaded portion of the shell
is called reach , and it’s
usually measured in inches. Common reach
lengths are 1/2-inch and 3/4-inch, although
there are many other reaches. You can install
a plug with incorrect reach, but you can
also really screw up an engine by doing
so. So always put a plug in with the
correct reach. Period.
Inside
the metal shell is an insulator ,
usually white, which is tapered at the
working end of the plug. The tapered part
of the insulator is called the nose .
Right
in the center of the insulator, is an electrical
conductor which connects the electrical
terminal at the top of the plug to
a round wire that sticks out of the insulator
nose at the firing end of the plug.
This
round wire is called the center electrode .
The main purpose of the insulator is to
prevent the electrical spark current from
going anywhere except down to the center
electrode.
Welded
to the bottom of the shell is a small piece
of metal called the side electrode ,
which sticks over toward the center
electrode and forms an air gap between the
two electrodes. The ignition current is
supposed to jump across the air gap between
electrodes and make a spark. The name of
the air gap is spark gap.
HOT
PLUGS VS COLD PLUGS?
Just
about everyone has heard that plugs have
heat ratings, and are called “hot” or “cold”
types. Some people think that means the
plug either makes the engine run hot or
cold, which is completely wrong.
The
amount of heat a plug brings into an engine
due to the electrical sparks is so small
it’s insignificant. The amount of heat
a plug removes from an engine is also
small.
Those ratings - “hot”
and “cold” - mean simply this: a plug becomes
very hot in an engine, so we say it’s a
hot-running plug — or just hot. Another
plug doesn’t reach such a high temperature,
so we say it’s a cold-running plug — or
just cold.
The
firing end of a hot plug reaches a higher
temperature than a cold plug, in the
same engine, under identical operating conditions. The
metal shell of each plug will operate
at very nearly the same temperature as
the metal of the head itself, because
the plugs are screwed into the head and
there is a good path for heat flow between
shell and head.
Heat
gathered by the insulator nose tends to
accumulate there, because the insulator
material is not a very good conductor of
heat. The path for heat flow is away from
the insulator nose. Heat has to flow upwards
along the nose until it reaches the place
where the insulator is in mechanical contact
with the shell.
From
there, heat can flow into the shell, then
to the metal of the engine.
If
the path for heat flow is short and relatively
easy, more heat will escape and the nose
of the plug will tend to run cooler.
If
the path for the heat flow is long and difficult,
more heat will remain at the tip of the
insulator and the plug will run hotter.
The main difference between hot-running
and cold-running plugs is the length
of the insulator nose.
Those
two electrodes also get hot. The center
electrode gets hotter than the side electrode,
because the center one loses heat to an
insulator, whereas the side electrode loses
heat directly to the metal shell of the
plug.
Spark
plugs are manufactured with different heat
ratings, from very cold to very hot, so
a suitable plug can be found for your engine,
depending on what you need, based on your
riding or driving conditions. Plugs with
the same diameter and reach will have different
lengths of the insulator nose section and
different type numbers to indicate which
runs hot and which runs colder.
These
plugs are mechanically interchangeable, but
will run at different operating temperatures
in the same engine. Part of the tuning
problem is to find a plug that survives
in an engine.
WHAT
HAPPENS WHEN THE PLUG IS TOO HOT?
It’s
easy to overheat a plug in an engine. A
mixture that’s too lean will do it, because
the gasoline drawn into the firing chamber
has a cooling effect. If there is not enough
gasoline, there is not enough cooling.
Working
the engine hard at slow speeds — such as
running uphill in a deep sandwash — will
do it, because the engine is generating
a lot of heat but airflow for cooling is
reduced by the slow speed of travel.
Using
the wrong spark plug will also do it also.
If you are not using the plug specified
in your owner’s - manual, you
might have problems.
When
a spark plug gets too hot, the insulator
may boil and bubble On examination, it
will be plain that it has been too hot.
Also, the metal electrodes may melt away
and disappear. If any of these bad things
happen, that’s
good.
The
worst result of a too-hot plug that fails-to
destroy itself, is when it destroys the
engine instead; this is called pre-ignition.
If the tip of the plug becomes hot enough
to ignite the fresh mixture being drawn
into the cylinder, then the incoming mixture
will start to burn without waiting for the
spark to happen.
Ignition
due to any hot spot in the cylinder begins
before the proper time for ignition,
so it is called pre-ignition. Anything in the
combustion chamber which gets hot enough
can cause pre-ignition, but typically the
end of the spark plug is the cause. When
the mixture is firing sooner than it should,
that’s like advancing the spark too much,
and no matter what causes it, early ignition
makes engines heat up, causing pre-ignition.
Eventually, something melts, which comes
under the heading of a bad thing.
WHAT
HAPPENS WHEN THE PLUG IS TOO COLD?
You
can install a plug which is too cold-running
for the engine. Or you can have an engine
that’s running way too cool, such as
riding ten miles downhill on a frosty
morning in the mountains.
Whatever
the cause, if the nose of the plug is not
hot enough, it will gradually accumulate
deposits, known as fouling. During
normal engine operation, residue from the
combustion process hits the insulator nose.
This may include carbon, unburned fuel and
oil, and chemical additives present in both
fuel and oil.
If
the insulator nose and electrodes are hot
enough, the combustion deposits will be
continuously burned off by the heat of the
plug. The ideal situation is to have the
deposits burned off as fast as they accumulate,
so the insulator nose stays fairly clean
and free of deposits.
If
the deposits accumulate on the plug because
it is not reaching a high enough temperature
to burn them off, the gradual accumulation
will eventually short out, or foul, the
plug. The fouling is electrically conductive
and forms a path along the insulator, which
connects the center electrode to the metal
shell of the plug.
The
spark current will flow along the fouling
path and will not jump across the
spark gap . The engine stops running,
but all you have lost is a spark plug. Fouling
due to a too-cold plug is much better than
pre-ignition due to a too-hot plug.
PICKING
THE RIGHT PLUG
If
the tip of the plug gets hotter than about
1500 degrees Fahrenheit, electrode burning
and pre-ignition will result. Below 600
or 700 degrees, fouling will occur. A
normal spark plug gets hot enough just
above idle to burn away fouling deposits,
and when running at full throttle, the
plug is still not hot enough to cause
problems. The reason a plug gets hotter
at full throttle is simply because more
fuel is burning in there.
A
plug which is too hot will run hotter than
normal over the entire range, and at
full throttle, will be into the pre-ignition
zone. Similarly, a plug which is too cold
tends to foul more easily when the engine
isn’t working hard.
Reading
the plug from your engine gives you some
clues about whether it is too hot or too
cold.
READING
THOSE SPARK PLUGS!
To
read a plug, you must first prepare it
properly for reading, so it will give
you the real story. That means you can’t
just go over to an engine and yank the
plug. You have to know how the machine
was operated in the period just before
you look at the plug, and the machine
must be run in some steady condition
for a while before taking the reading.
You
can’t just make a fast pass down a dirt
road, then pull into the pits and let the
engine idle for a while before shutting
it off. This kind of a reading from a spark
plug will be totally worthless. A plug
can tell you only about a recent, sustained,
single operating condition.
First
off, the engine should be in reasonably
good tune and mechanical condition, and
the plug should also be in reasonable
condition and the plug proper for the
engine.
Then
you can begin by making a flat-out, full-
throttle run up a slight incline for
about a mile. If you try that with a
worn engine, or
in bad tuning condition, it may not last
the flat-out mile.
But
if it does, at the end of your run, pull
in the clutch and kill the ignition.
Coast to a stop and don’t run the engine
anymore until you yank the plug. That
is called cutting clean. What the plug
will now tell you is how things were
inside the engine at full throttle.
The
best way to read the plug after removing
it from the engine is to use an illuminated
magnifier. With that, you can see small
details and get more information. The
average person just looks at the plug
with his natural eyeballs and learns
some things, but not as many things as
with a magnifying glass.
The
main indicator is the general appearance
of the insulators. If it looks like it has
been too hot — it has. If it’s showing some
deposits, chances are it’s too cold.
Now
you’re ready for a second plug reading,
and this one should be taken at mid-throttle.
The full-throttle test tells you about the
condition in the combustion chamber while
it was on the main jet, because that’s
what controls mixture strength at full
throttle.
The
half-throttle reading tells you about the
needle and needle jet (or mid-range jets)
performance, because they control that part
of the engine performance. Most average
tuners won’t even perform this part of
plug reading and tuning, and will be happy
with full throttle runs and readings.
READING
AND CHANGES
If
you yank a plug and find a nice off-white
or slightly tan/gray color with new-looking
electrode shapes and no evidence of problems,
the thing to do is smile real big and
put the back in
If
you find black fouling to suggest a too-cold
plug, or the plug is burned white and melty
looking, you better do something to correct
the situation. You can certainly change
plugs, provided you have the recommended
normal plug in the engine. Most manufacturers
select the normal plug for normal riding
conditions. You can go colder or hotter,
or you may even have to make carburetion
or timing changes. We can’t get into all
that here, but you get the drift.
If
your wife or girlfriend just putts around
all day, they aren’t making enough heat
to keep the plug clean. This is the perfect
condition for you to use the next hotter
heat rating to see if it helps. You find
the type number of the next hotter plug
by checking a spark plug selection chart,
or by asking at the parts counter.
If
the plug looks burned and you have been
riding all day flat-out, tucked-in and
too fast, you should more than likely
to go one step colder in plug heat rating. If
you don’t have the correct plug installed,
or its next-door neighbor on the heat rating
chart, your best bet is to try the correct
plug first and then go from there.
If
you can’t solve the problem by a one-step
heat rating change of spark plugs, you
may need to rejet the carb, or do mechanical
work on the engine.
PLUG
VARIATIONS
You’ll
find all kinds of plugs, plug materials,
designs and variations. And you’ll also
find wild claims and promises to match. Most
plugs have what is called the conventional
gap style, and it doesn’t have any letter
identification. It is the common plug used
in cars and motorcycles. There are zillions
of these around, giving good service.
In
the I-gap, the side electrode is cut back
partially so it only coversabout half of
the center electrode. This design
is less subject to foulingdue
to something getting between the electrodes. This
is often considered to be a special design
for two-stroke engines.
A
Y-gap, or extended-nose plug, has a wider
heat range than a conventional plug, which
means it operates well over a greater range
of temperatures. The insulator nose and
gap will stick out pretty far into the
combustion chamber when this plug is installed.
Because of the extended nose, the tip of
the plug is farther out into the combustion
area and receives more heat at low engine
speeds. This tends to prevent fouling at
low operating temperatures.
At
open throttle where you might expect this
plug to heat up more than a conventional
design, the extended nose gets a good blast
of cool, fresh mixture during each intake
and therefore it gets less hot than a conventional
plug. This _ extends the operating range
to higher engine temperatures. This is
normally considered to be a four-stroke
plug.
An
extended-nose plug cannot be casually substituted
for a conventional plug type, because
the extended nose may get hit by the
piston, or a valve. If it is specified
as original equipment, you know there’s
enough clearance for the long
nose.
A
G-gap plug is also called a fine-wire plug,
because the electrodes are made of smaller
wire. A spark will jump at lower voltage
across a gap formed of sharp edges or
smaller electrodes. Both the I-gap and
G-gap benefit from this. The J-gap “shows” the center
electrode the sharp edge of the side electrode
and that gives a lower firing voltage requirement.
The
Gold Palladium fine-wire design has the
same advantage, because the electrodes are
smaller. When the electrodes are made smaller,
precious metals such as gold and platinum
are better able to withstand heat and erosion.
G-gap plugs cost more, but when properly
used, they are worth it.
A
surface-gap plug uses a completely different
construction, with no side electrode at
all. An insulator fills the space between
center electrode and shell. The spark jumps
from center electrode directly to the shell,
taking any path it chooses. These plugs
are for motor-cycles with super-high-voltage
electronic ignition and should only be used
where they were originally. specified by
the manufacturer. |
