15
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6.5.2.1. Spark plug chart
Technical data
EG4-0210
EG4-0420
Thread
M14x1.5
Length
19,0 mm
Electrode distance
0,7 mm
Manufacturer
BONON
F7TC
F6RTS
BOSCH
W7DC
CHAMPION
N9YC
NGK
BPR6ES
6.5.2.2. Appearance of the spark plugs
Normal state
This spark plug is in good state. The insulator is gray-
ish-white to yellowish-gray. The plug is working under
optimal temperatures. Here the heating value, the igni-
tion and the alloy are well adjusted, the electrodes are
not worn, they only have little residue and the electro-
des have the perfect distance.
In the past the ideal colour of the spark plugs
was to be brownish-red. This goes back to
the time when lead was added to the gasoli-
ne and is not the case nowadays.
Worn electrodes
In this picture you see extreme burn-off of the elect-
rodes. This leads to bad correspondence with the mo-
tor, as well as problems starting up. Although there is
barely any residue and the plug has the right colour
you will have to replace it. Actually, it would have been
necessary long ago! Use a spark plug with the same
heating value as the previous one, which corresponds
to the specification in this manual.
Soot residue
In this picture you see sooty residue. This is not dange-
rous, indicating a too heavy fuel air alloy. This residue
is particularly prominent when the motor is used with a
light load. After a few minutes of using the motor with
its maximum load the residue should pass. If it does
not the heat-value of the spark-plug is incorrect (wrong
spark plug). Also, the air filter could be dirty or clogged,
the valves lose or the choke not fully deactivated.
Spilt/glass-like residue
Small dashes of residue indicate that there is dirt in the
intake, which is sucked in and melts. The problem is
solved by cleaning the carburetor and replacing the air
filter. A yellowish, glass-like shiny glaze accumulates
on the tip of the insulator from gasoline or motor oil
residue and melt during maximum load. The resulting
problems with ignition should be avoided during pro-
longed use with lower power.
Combustion residue
These light brown crusts on the electrode and the in-
sulator are caused by residue from gasoline and mo-
tor oil. The spark plug does not work properly and the
combustion gets worse. Often larger amounts of oil get
into the combustion chamber, are not burnt entirely and
leave behind this residue. Check the valve guide and
valve seat and do not use gasoline additives. Changing
the brand of gasoline you are using can also help.
Bridged electrodes
In older motor cycle times, then the fuel was low in
quality, this was a common problem. The residue ac-
cumulated to a kind of bridge between the electrodes.
Two-cycle engines were particularly prone to this pro-
blem. Here cleaning the spark plug helps, as well as
using high quality fuel and oil. Of course, there might
be a more deep-seated problem such as a worn piston
or worn cylinders.
Overheating
The insulator is chalky white and there is absolutely no
residue visible. The motor got way too hot! The cause
could be a too low heating value of the spark plug or
a too light fuel air alloy. Check all of these points and
replace the spark plug with a new one according to the
chart.
Oil residue
Oily and greasy residue is an indicator of large amounts
of oil getting into the combustion chamber. The cause
of this could be a problem in the valve guide, a worn
piston, piston ring or cylinder liner. Checking the com-
pression can help find the problem. Usually this causes
a greater engine overhaul. In two-cycle engines this
state can also occur when the crank case seal fails and
gear oil is sucked in.
Gasoline residue
The plug is very similar to the one depicted above.
When removed however, the wet spots dry quickly and
only soot residue is left. This is not oil but fuel. The mo-
tor was flooded because there was too much fuel in
the combustion chamber. Here cleaning the spark plug
and opening the choke helps. If there still is a problem
check the ignition and the needle valve.
Broken insulator because of back fire
When the motor backfires hard the insulator can break
and even spall. Check whether you used gasoline with
the correct octane value. A problem in the cooling sys-
tem or an incorrect distance between the electrodes
can cause problems. Other causes are bad ignition ti-
ming (too early) or too light fuel air alloy (additional air
or valves clogged). A wrong spark plug (wrong heating
value) can cause this also.
Molten electrodes because of a
knocking engine
If the electrodes have molten, uncontrollable ignition
and very high temperatures were the cause. The wrong
heating value, overheated valves, light carburetor set-
tings, additional air, too much carbon residue and sharp
edges in the combustion chamber could be the cause.
Check all of these points and replace the spark plug
according to the chart.
Electrodes and insulator molten
If you ignore engine knocking for a longer amount of
time the electrodes and even the insulator will melt.
The wrong heating value, light carburetor settings, way
too early ignition, additional air, too much carbon resi-
due and sharp edges in the combustion chamber could
be the cause. Check all of these points and replace the
spark plug according to the chart.
Mechanical destruction
Rare, but it has happened! This spark plug was comple-
tely destroyed because it touched parts in the combus-
tion chamber. It is extremely important to find the cause
of this. Maybe the piston or the valves are broken, or
large amounts of carbon residue got loose. However,
you might have used a spark plug that was too long
and touched the valves of the piston. Check all of these
things and replace the spark plug with a new one.
