Reading spark plugs, how to read you ultralight aircraft engines spark plugs.

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Reading the spark plugs of your ultralight engine.

A lot can be learned about your Rotax engine condition by properly reading and "understanding" your spark plugs.

This page will show you pictures as well as explanations on cause and effect of what you see when viewing your spark plugs.

Look at the spark plug porcelain for the color and condition of it. Plug colors can tell you how the engine is running and can be very important. Plugs change colors and the different colors can explain such things as if the engine is running too hot or if the engine is worn.

The color should be read by looking at the porcelain insulator and compared to the plugs shown on the following pictures.

A tan coloured 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 helps the plug fire better.

This plug is worn out from being used for a long period of time. Notice how the center 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.

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. Make sure the spark plug is the correct length for the engine.

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.

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).

Carburetor 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.

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.

This plug is oiled fouled, caused by poor oil control.

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.

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.

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.

A black dry fluffy coloured 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 causes for a rich air/fuel mixture are:

dirty air filter.

air mixture screw or carburetor needs adjusting.

choke is sticking.

carburetor float 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 will tend to elevate combustion chamber temperatures to unsafe levels, and pre-ignition and detonation will usually follow. Serious engine damage is not far behind.

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...in either case, heat transfer can again be slowed and the above mentioned conditions can occur.

The spark plug holes must always be cleaned prior to installation, otherwise you may be torquing against dirt or debris and the spark plug may actually end up under-torqued, even though your torque wrench says otherwise. Of course, you should only install spark plugs in a cool engine, because metal expands when its hot and installation may prove difficult. Proper torque specs for both aluminum and cast iron cylinder heads are listed below.

Spark plug type
Thread Diameter
Cast Iron Cylinder Head (lb-ft.)
Aluminum Clyinder Head (lb-ft.)
Flat seat type
(with gasket)
18 mm
25.3~32.5
25.3~32.5
"
14 mm
18.0~25.3
18.0~21.6
"
12 mm
10.8~18.0
10.8~14.5
"
10 mm
7.2~10.8
7.2~8.7
"
8 mm
--
5.8~7.2
Conical seat type (without gasket)
18 mm
14.5~21.6
14.5~21.6
Conical seat type (without gasket)
14 mm
10.8~18.0
7.2~14.5

Gapping

Since the gap size has a direct affect on the spark plug's tip temperature and on the voltage necessary to ionize (light) the air/fuel mixture, careful attention is required. While it is a popular misconception that plugs are pre-gapped from the factory, the fact remains that the gap must be adjusted for the vehicle that the spark plug is intended for.

Q: How often should I replace my spark plugs?

A: Unfortunately, there is no single answer to this question. As spark plugs grow older, they lose their sharp edges as material from the center and ground electrodes is slowly eroded away. As the gap between these two points grows, the voltage required to bridge the gap increases proportionately. Even the best ignition systems will be strained to supply enough voltage to completely burn the fuel. It is at this point, when fuel is being left unburned, that the time has come to change spark plugs.

Replacing worn out spark plugs with new ones (with sharp new edges) effectively restores the ignition system's efficiency. Misfires are reduced, power is restored, economy of operation is enhanced and emissions are reduced.

The best guide is the manufacturer's recommendation for your vehicle, as this particular service varies from brand to brand and model to model. In the absence of this information or in conjunction with it, you can rely on the advice of a mechanic who is familiar with your type of vehicle. In the best of all worlds, this would be a mechanic who is also familiar with the vehicle you own. If you find a good mechanic, whether dealer or independent, stick with him. The better he knows your personal vehicle, the better he will be able to diagnose and service it. The end result is very much like a doctor-patient relationship and, in the long run, you will have a healthier vehicle.

Q: What do the numbers and letters in a part number represent?

A: The various numbers and letters in a spark plug code or identification number basically identify the features and functions of a particular plug. Included would be information such as plug type, heat rating, construction, thread diameter, thread reach and firing end construction.

Click on the link below to download a guide that will help you decipher the various NGK alpha-numeric combinations. You will need to open it with Adobe Acrobat Reader. If you do not already have Acrobat Reader installed on your computer, we've provided a link to the free download area for your convenience.

Download the part number guide
Download Adobe Acrobat Reader (Free)

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