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

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.

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.

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.

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)