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Reading Spark Plugs

Views: 0     Author: Site Editor     Publish Time: 2022-04-06      Origin: Site

Reading Spark Plugs 

Pictures and explanations (cause and effect) to be used in visual inspection of spark plugs.

The spark Plug insulator color and condition can indicate how the engine is running. Color changes indicate such things as combustion temperatures or engine condition.

1 A tan colored plug means that the engine is running normal and the air/fuel mixture is correct. This is the correct color a spark plug should be and it tells you everything is fine with the engine. You would install a new properly gapped plug. When installing a new plug, replace the old one with the same heat range. This plug shows normal wear in the center electrode. A new plug would have square edges that improves ignition.
2 This plug is worn out from being used for a long period of time. Notice how the center electrode is rounded worn from use. A spark plug that is worn requires more spark voltage to fire and can cause engine misfires.
3 This plug 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. Verfiy the spark plug is the correct length for the engine.
4 Excessive detonation has caused the porcelain on this plug to break away. If this engine is allowed to run, engine damage can occur.  Low fuel octane can contribute to this condition.

A white colored plug is caused by engine overheating. Failure to repair this engine will result in severe engine damage. common causes for this are

1, Incorrect spark plug (Too hot heat range).

2, Low octane fuel

3, Timing is not set properly

4, Cooling problem, (dirty cylinder fins, no or low water if water cooled, low or no engine oi)

5, Carburetor air/fuel mixture is too lean (too much air)

6, Leaking crankshaft seals, no oil, base or head gasket leaks, or crankcase leaks on two stroke engines

6 This plug has ash deposits which are light brownish deposits that are encrusted to the ground and/or center electrode. This situation is caused by the type of oil used and adding a fuel additive. This condition will cause a misfire. This can be also caused by changing oils in midstream.
7 This plug is oiled fouled, caused by poor oil control.
8 Pre-ignition, which will usually look as a melted center electrode and/or ground electrode. Check for incorrect heat range plug, over advanced timing, lean fuel mixtures or even hot spots or deposit accumulation inside the combustion chamber.
9 Sustained Pre-ignition, which will usually look as a melted or missing center electrode and/or ground electrode as well as a destroyed insulator. Check for incorrect heat range plug, over advanced timing, lean fuel mixtures or even hot spots or deposit accumulation inside the combustion chamber.
10 Splashed deposits look as if they are small islands of contaminants on the insulator. This is usually caused by dirty carburetor bores or air intake.
11 A black dry fluffy colored plug is caused by deposits from a carburetor 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 cause for a rich air/fuel mixture are:
1, dirty air filter
2, air mixture screw or carburetor needs adjusting
3 choke is sticking
4, carburetor floating height is out of adjustment or float is sticking open

Spark plug installation:

Torque is one of the most critical aspects of spark plug installation. Torque directly affects the spark plugs' ability to transfer heat out of the combustion chamber. A spark plug that is under-torqued will not be fully seated on the cylinder head, hence heat transfer will be slowed. This elevates combustion chamber temperatures to unsafe levels, pre-ignition, and detonation will usually follow. Serious engine damage is possible.

An over-torqued spark plug can suffer from severe stress to the Metal Shell which in turn can distort the spark plug's inner gas seals or even cause a hairline fracture to the spark plug's insulator. Either condition causes heat transfer issues as defined above.

The spark plug holes must be cleaned prior to installation, otherwise you may be torqueing against dirt or debris and the spark plug may actually end up under- torqued, even though your torque wrench says otherwise. You should install spark plugs in a cool engine, because metal expands when its hot and installation may prove difficult.  Refer to proper torque specs for engine.


The gap size has a direct effect on the spark plug's tip temperature and on the spark voltage necessary to ignite the air/fuel mixture. Verify plug gap.

Spark Plug Numbering

Plug number and letter codes identify the features and functions of a particular plug. Plugs are defined by type, heat rating, resistor type, thread diameter, thread reach and firing end construction.

Example: Brush cutters often use a higher heat range plug to reduce plug fouling, caused by over-rev protection (ignition cut-out).

Replacing worn out spark plugs with new ones (with sharp new edges) effectively restores the ignition system's efficiency. Misfires are reduced, power restored, economy of operation is enhanced and emissions reduced. In conjunction with the operators manual, rely on spark plug inspection to determine when to change spark plugs.

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