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Spark Plug Diagnostics
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These pictures and descriptions were taken from an
Australian dirt bike website, which appears to no longer exist. I have had
these "on file" for some time and I publish them, with thanks,
unchanged.
Look at the spark plug porcelain for the colour and condition of
it. Plug colours tell the mechanic how the engine is running and can be very
important. Plugs change colours and the different colours can explain such things
as if the engine is running too hot or if the engine is worn. The colour should
be read by looking at the porcelain insulator and compared to the plugs shown on
the next few slides.
| A tan coloured plug means that the engine is
running normal and the air/fuel mixture is correct. This is the correct colour
a spark plug should be and it tells the mechanic everything is
fine with the engine. The mechanic would install a new properly gapped
plug. When installing a new plug, replace the old one with the same heat
range. The plug below shows normal wear in the centre electrode. A new
plug would have square edges that helps the plug fire better. |
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A black dry fluffy coloured plug is caused by
deposits from a carburettor that is running too rich (too much gas), or
excessive idling on some engines. Black smoke coming from the exhaust is
a sign of a rich air/fuel mixture. The rich air/fuel mixture must be
repaired before installing a new spark plug. Common causes for a rich
air/fuel mixture are:
- dirty air filter.
- air mixture screw or carburettor needs adjusting.
- choke is sticking.
- carburettor float height is out of adjustment or float is sticking
open.
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A black wet glazed coloured plug is caused by
the burned oil that is leaking past the piston rings or valve guides and
burning in the cylinder. This engine will have blue smoke coming out of
the exhaust. It is a sign that the engine will need rebuilding. Two
strokes that have this colour spar plug are burning the transmission oil
from the crankcase. If a new spark plug is installed it will foul and
quit working. Common causes for this problem are:
- Worn piston and rings.
- Worn cylinder bore.
- Worn valve guides.
- Crankshaft seals are bad or cases that need lapping (if on a two
stroke engine with a transmission).
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A white coloured plug is caused by engine
overheating. Failure to repair this engine will result in severe engine
damage. Common causes for this are:
- Incorrect spark plug (too hot heat range).
- Low octane fuel.
- Timing is not set properly.
- Cooling problems, (dirty cylinder fins, no or low water if water
cooled, low or no engine oil).
- Carburettor air/fuel mixture is too lean (too much air).
- Leaking crankshaft seals, no oil, base or head gasket leaks, or
crankcase leaks on two stroke engines.
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| The plug below shows what can happen when
something hits the spark plug. Something from inside the engine has hit
the plug and this problem must be repaired before running the engine
further. Make sure the spark plug is the correct length for the engine. |
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Excessive detonation has caused the porcelain
on this plug to break away. If this engine is allowed to run, engine
damage can occur. Make sure the fuel octane is high enough for the
engines requirements. |
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| The plug below has carbon across the plug
gap. This is usually caused by carbon that is loosened and it lodges
between the gap causing the plug to misfire. |
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The plug below was caused by pre-ignition and
failure to repair this condition will cause engine damage. Check for the
following:
- correct heat range plug
- over advanced ignition timing
- lean air/fuel mixture
- cooling system
- lack of lubrication
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| The plug below is worn out from being used
for a long period of time. Notice how the centre electrode is round and
worn from use. A spark plug that is worn takes a lot more voltage to
fire and can cause poor engine running. |
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NGK Spark Plug Codes:
NGK code: [B] [P]
[6] [E] [S]
Field one: Thread Diameter
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A
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18 mm
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B
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14 mm
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C
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10 mm
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D
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12 mm
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Field two: Construction
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C
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Hex size 5/8"
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K
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Hex size 5/8" with projected tip (ISO)
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M
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Compact type
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P
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Projected insulator type
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R
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Resistor
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SD
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Surface discharge for rotary engines
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U
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semi-surface discharge
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Z
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inductive suppressor
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Field three: Heat Range
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2
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Hot
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10
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Cold
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Field four: Thread reach
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E
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19 mm
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F
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tapered seat
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H
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12.7 mm (1.5")
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L
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11.2 mm (7/16")
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If this field is blank, an 18 mm diameter plug has 12
mm reach; and a 14 mm plug has 9.5 mm (3/8") reach.
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Field five: Firing and construction
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A,B
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Special design (no details given)
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C
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special ground electrode
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G
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Racing use
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GV
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Racing V-type
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H
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Half thread
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K
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2 ground electrodes
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L
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Half heat range
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LM
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Compact lawn mower type
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M
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2 ground electrode for Mazda rotary engine
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N
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Special ground electrode
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P
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Platinum tip (premium)
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Q
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4 ground electrodes
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R
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Delta ground electrode for BMW
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S
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Standard 2.6 mm electrode
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T
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3 ground electrodes
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V
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Fine-wire electrode, gold-palladium
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VX
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Platinum tip (high performance)
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W
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Tungsten electrode
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X
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Booster gap
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Y
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V-groove centre electrode
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Field six: Wide gap
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8
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.032"
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9
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.036"
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10
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.040"
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11
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.044"
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13
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.050"
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14
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.055"
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15
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.060"
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20
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.080"
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