In part 1, we discussed the theoretical aspects of the spark plugs. In part 2, we talked about the more practical aspects of installation and maintenance of the spark plugs installed in the Rotax engine. In this, part 3, we are going to take a look at using the spark plug as a troubleshooting tool.
The engine will continuously communicate her condition to you through a multitude of sources. One of the most powerful sources of information about an engine’s internal condition comes from what the spark plugs have to tell you. We call this “reading” the spark plug. (Figure: 1) One advantage of reading the spark plugs is that they don’t often lie. Engine instrumentation can give us a lot of clues about what’s going on inside the combustion chamber, but they are limited, both in the scope and in their accuracy. We see aircraft owners pulling their hair out, modifying the engine, trying to make it operate in a configuration so that the instruments read “normal”, only to find out months later that their instrumentation has been indicating incorrectly all along. By this time, they have modified the engine so far from the stock configuration that the road back to proper operation if often frustrating and expensive.
When we talk about reading the spark plug, we are primarily talking about reading the color of the ceramic insulator that surrounds the center electrode. The automotive industry, and in particular the racing industry, has carried this reading of the spark plugs to a very fine, nuanced science. And although engine to engine readings vary substantially, we can take away from some general principles that you should be able to apply to your particular engine. It is common that we can get information about all segments of the engine operation from idle to full throttle by looking at selected segments of the spark plug. Idle operation can be revealed by looking at the face of the threaded spark plug body. Midrange, the place where you spend most of your operating time on the engine, shows itself primarily towards the end of the ceramic insulator where the center electrode protrudes. And full throttle operation is more indicative of a ceramic insulator deep inside the area where the ceramic makes contact with the spark plug body. Now I caution you, these principles need to be associated with a significant understanding of the operating characteristics of your particular engine. Even the visual appearance of a Rotax 582 two-stroke engine versus a Rotax 912 four-stroke engine under normal operating conditions will have a substantially different appearance.