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SPARK PLUGS
People who are relatively inexperienced in the art of tuning racing two-stroke engines seem always to
all into the trap of trying to use a spark plug that is too "cold", and / or one having the wrong nose
configuration, which they then compensate with a too-lean mixture. This kind of error probably stems from a
basic misunderstanding of spark plug heat-range, and the reason for having more than one type of plug. The
reason?
As it happens, the temperature of the burning mixture in the combustion chamber is high enough to melt the
Engine, and it fails to do so only because heat is carried away from the combustion chamber's walls fast
enough to prevent them from reaching flame temperature. The spark plug reaches much higher temperatures
than the cylinder head itself, because heat moving away from the plug's nose must cross the joint formed, by
the threads on the plug and in the plug hole. And, of course, the exposed plug electrodes are separated from
the cylinder head's cooling fins by -a very long heat-path, and in the case of the center electrode, by the plug's
insulator. Consequently, the electrodes get extremely hot, and that is very much a mixed blessing: Fairly high
temperatures are required to keep the plug's electrodes and insulator tip burned clean, to prevent the
formation of sooty deposits that can short-circuit the spark. But if the electrodes are allowed to become too
hot, they will constitute an independent and uncontrolled ignition source. That is to say, the electrodes
may
become hot enough to ignite the air/fuel mixture in advance of the spark. When that occurs, combustion
chamber temperatures climb even higher which causes auto-ignition still earlier and that yields yet higher
temperatures in an ascending spiral that quickly leads to outright
Engine failure.
So, the correct spark plug for a given Engine is one that stays hot enough to burn its electrodes and
insulator tip clean, but does not reach temperatures high enough to cause auto-ignition (more commonly
termed, "pre-ignition"). Unfortunately, all spark plugs transfer heat from their electrodes to the cylinder head
into which they are threaded at a rate fixed by their construction, while Engine operating temperatures vary
enormously. "Correct" selection of a spark plug is therefore a matter of choosing one having a heat-transfer
rate compatible with the application at hand. The process of selection is less exacting today than in the early
days of the
internal combustion Engine, principally because the mica and porcelain used as insulators have
been replaced by aluminum oxide ceramics that are stronger and much better conductors of heat. Other
improvements in spark plug construction have further widened plug heat-range, hut not to such extent that
any plug will perform satisfactorily in any Engine. Touring plugs have extended tips, which burn clean but
have exposed ground electrodes and center electrodes, and these respond to prolonged full-throttle operation
by becoming incandescent; racing plugs have short, shielded electrodes and will tolerate higher combustion
chamber temperatures but foul very quickly under starting and idling conditions.
The most common spark plug configuration is the one in which the ground electrode extends over the
end of the center electrode. A much better type, in any two-stroke Engine application, has the ground
electrode reaching in slightly lower to approach the center electrode from its side. The improvement, here, is
that the ground electrode is somewhat shorter, and being shorter offers a more direct heat path to the plug
body - which means that it is less likely to become white-hot. A refinement of this design has a short bit of
platinum wire inserted in the tip of the ground electrode. This wire's diameter is quite small, and it is used in
conjunction with a center electrode also reduced in diameter at its tip, an apparently minor difference unless
you consider that much less voltage is required to form a spark between two points than between two flat
surfaces. An ordinary iron ground electrode cannot be made pointed, because its tip would instantly overheat
and melt, but platinum -with a melting point of 1774 C, as compared with 1535 C for iron - is less likely to
melt, has a thermal conductivity 18 times greater than iron, and will not oxidize. As a result, the platinum
electrode survives its hostile environment very well even when used in small diameters, and I would not
hesitate to recommend the platinum-tip plug for most racing applications. At any given heat-range, the
platinum plug will provide longer life and less tendency to foul than any conventional type plug. Pure racing
plugs, which have deeply recessed insulators and center electrodes, with an extremely short ground electrode
bridging straight across from a hole through the side of the plug body, should be used only as a last-resort in
two-stroke engines. Racing plugs of the type described are very, very prone to wet fouling, oil fouling and
every other kind of fouling possible. They are a necessary evil in hyper-horsepower road-racing engines, but
should never be used where extended-nose plugs will serve. Not unless there is some kind of major
breakthrough in either spark plug or ignition system design, which always is a possibility. I would recommend
that all who are serious about keeping ahead of the pack keep abreast of developments in these areas. All the
spark plug manufacturers are working constantly to give us improved foul resistance and reduced spark-
voltage requirements, and they are very accommodating to anyone who takes the trouble to write and ask for
literature.
"Reading" spark plugs, and the process of selecting correct heat-range, falls much more into the realm
of art than science, and it is an art in which one becomes really proficient only after long and sometimes
painful experience. But there are a few rules that may be used for guidance by those who have yet to acquire
experience - or by the many whose experience has left their ignorance largely undiluted: First, you should
know that it is all but impossible to read anything in the appearance of a spark plug unless the Engine has been
cut clean after having been brought up to operating temperature and given a long burst of wide-open throttle.
Very experienced tuners will see the signs they're looking for under the layer of soot, oil and fuel that
'accumulates so quickly at idle, but even they vastly prefer to work with clean-cut plugs. Second. get the right
heat-range before you try to rend mixture strength, and my recommendation is that you always use the hottest
plug the Engine will tolerate. You'll know a plug is too hot when you observe signs of blistering around the
insulator nose (which will also be scorched white) and on the electrodes. A too-lean mixture will also give
you a whitish insulator, but will not usually produce the burned, pitted appearance of the electrodes that is
characteristic of a too-hot spark plug. Also look for signs of melting along the sharp edges at the ground
electrode's end – any sharp corner will get hotter than other areas along the electrode, and trouble will first be
revealed there. A plug that is too cold simply looks, and is, wet. Plugs of the correct heat-range get hot
enough to burn away oil, and soot, and will have only dry, brown to tan deposits on their insulators after a
hard run. As noted before, the correct mixture strength will be wry slightly leaner than that which is just lean
enough to keep the Engine from four-stroking. How much leaner? Not very much, and until you have gained
considerable experience with a particular Engine you should not reduce strength below the jetting that
provides clean running. To get a bit closer to the optimum, I watch the faint, almost invisible ring of soot that
forms around the electrode on the insulator's nose, and the light dusting of soot over the exposed end of the
plug body. There is a point at which I see "just enough" soot, and if there is more or less than that - I interpret
the signs to mean a mixture that is too rich or too lean, respectively. And I couldn't begin to tell you how
much soot is "just enough”; that would be like trying to explain a taste, or sound, or smell. With experience,
you learn to recognize what it is you're looking for, and there is no substitute for that experience. You will
also learn - if you know where to look - that the faint light spot on the electrodes at the sides of the spark gap
are an important clue to ignition system performance. When you have a spot of about the same diameter as
the ground electrode's end showing on the center electrode, or vice versa, then you may be sure the magneto
is doing its job. When that spot begins to fade, or become ragged around the edges and shrinking in diameter,
the ignition system isn't performing as it should. Finally, with experience you'll learn to give your very close
attention to all aspects of the mundane task of selecting jets and plugs, and spark timing, because in these
things you ultimately succeed or fail as a tuner; all the rest is mere mathematics, surgery and wrench-twirling.
2 Stroke Tuners Handbook by Gordon Jennings