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.
We can work with the generic principles that a hot/lean running engine will burn off the carbon deposits and leave a white ceramic insulator. And a cold/rich running engine will not completely burn the fuel deposits and leave dark black deposits on the ceramic insulator. A range of colors from white moving through the browns and onto black can give us a significant range of color that correlate with combustion chamber temperatures. Working with the Rotax 582 fuel consumption graph, (Figure: 2) we can correlate the engine temperatures with the engine (EGT) exhaust gas temperatures. The Bing carburetor is designed to provide a relatively rich mixture at idle. This rich mixture is necessary and normal at idle (2000 RPM). This is because the Bing carburetor does not incorporate an accelerator pump. The accelerator pump on a conventional carburetor is used to provide additional fuel during the throttle advance and acceleration process. The Bing carburetor eliminates this additional mechanical component and instead relies on the excessively rich mixture at the idle settings to provide the additional fuel for acceleration. As a result, we would expect to see a rich mixture on the area of the spark plug that garners idle information.
The Rotax 582, like many other engines, is designed to operate continuously around 75% power. If we look at the fuel consumption chart, we can see that this is the design point at which the engine is intended to get the most efficient fuel consumption. This will also be the leanest mixture and hottest EGT settings. And when we look at the ceramic insulator, near the center electrode, we will see the result of this leaner mixture. When operating correctly the insulator will garner a nice tan or light brown color. This process is so accurate that often times you will see variations in color around the perimeter of the ceramic insulator. This is normal and generally a result of the flame front travel emanating from each spark plug burning the fuel in a slightly different pattern as it crosses the combustion chamber. Now, if we follow the fuel consumption curve even further beyond the 5500 RPM cruise position, towards full throttle, we can see that the mixture now starts to increase and you will notice that the mixture gets significantly richer as we reach the full throttle position. This is designed into the engine/carburetor to utilize excess fuel at these higher power settings to act as a combustion chamber cooling agent. As a result, we should see the spark plug insulator turn to a darker brown color in the area that would indicate full throttle operation. Keep in mind that the more time spent at any one of these power settings, the more influence that those conditions will have on the other areas that we are trying to read. An hour’s worth of flight in the traffic pattern, doing touch and goes, generally will render a fairly good representation of idle, midrange, and full throttle operation, all on the same spark plug. Reading a spark plug after only a few minutes of operating time at any setting can be quite misleading.
Now that we have covered the basics of reading a spark plug, let’s talk a little bit more about some of the most common anomalous indications that we might see.
Carbon Fouling: (Figure 3) Carbon fouling is an indication of excess fuel within the combustion chamber. It will leave a dark, dry, sooty appearance on a portion or all of the spark plug. There are a multitude of potential causes for having an excessively rich mixture. Troubleshooting fuel related problems can be greatly enhanced by understanding which portions of the carburetor control which stages of throttle operation. (Figure 4)
Worn Needle Jets and Jet Needles. Even if you’re getting an indication that the EGT is starting to run a little cooler, it may be worth a look at the spark plugs. It’s very common for the needle jet and the jet needle to become worn to the point of causing an excessively rich mixture. This particular type of wear will show symptoms primarily in the midrange. It’s not a matter of whether you’re going to have this problem or not, but rather, when. Rebuilding the carburetors on the Rotax engine is a normal part of routine maintenance. Even though the Rotax manual specifies a very specific time interval for carburetor rebuild, the wear that dictates the necessity for the rebuild can be exacerbated by improperly synced carburetors, propeller vibration from imbalance or tracking, high moment of inertia propellers, continuous excessively low idle operation, improperly operating engine, or any other condition which causes the engine/carburetors to vibrate.
Improperly adjusted or sunken floats. Although it is not a new phenomenon to the Bing carburetors, Over the last couple of years we have been dealing with an increased prevalence of sinking floats, particularly on carburetors used on the Rotax 9 series engines. There are several service bulletins from Rotax that address this problem. Even improperly adjusted float arms can render the same symptoms.
Stuck or leaking needle and seat within the float bowl. A contaminated or corroded seat or a malfunctioning needle will raise the fuel level within the float bowl which will in turn enrichen the mixture across the entire spectrum of operation. The Viton rubber tip on the needle is a life limited part and is a common replacement item at carburetor overhaul. A malfunctioning seat as a result of damage or corrosion generally requires the replacement of the carburetor. These problems may also be accompanied by symptoms of fuel continuing to over-fill the float bowl after engine shutdown. Excessive fuel pressure from an auxiliary fuel pump can also cause the condition of off-seating the needle and seat. The Rotax maintenance manual contains a testing procedure for checking the integrity of the needle and seat. In addition, an engine that is not properly synchronized can also cause enough shaking, particularly at idle, to cause the needle seat to off-seat in the float bowl causing similar symptoms.
A stuck choke (enriching valve): Having a sticking choke cable is a very common occurrence. The return spring on the choke is a very lightweight spring and oftentimes does not have enough pressure to return the choke to its off position after release of the choke control. Additionally, if the enriching valve plate has contamination between the seating plate and the carburetor body, this will allow fuel to bypass the normal routing within the choke mechanism, acting as though the choke was in the on position even though the visual position of the choke cable is observed to be off.
Contaminated air filters: Failing to properly service the air filters can cause an excessively rich mixture.
Oil Fouling: (Figure 5) Oil fouling color is similar to fuel fouling however the difference is, with oil fouling the plugs are typically wet with oil whereas with fuel fouling the plugs are typically dry. As you might imagine oil fouling usually means we’ve got some serious work ahead of us.
Rings: Piston rings that are stuck, broken, or worn could be the culprit. This potential problem can usually be validated through a compression check.
Worn and leaking valve guides. An intake valve guide that is leaking is a different indication than that of an exhaust valve guide leak. Because the intake valve guide leaks into the intake manifold, all of that oil is routed through the combustion chamber and then out the exhaust. The intake valve guide may be suspect if the spark plugs are black, wet, and oily. This oil may be partially consumed in the combustion process and produce black soot in the exhaust and onto the exterior of the aircraft. On the other hand, an exhaust valve guide leaks directly through the guide into the exhaust manifold, bypassing the combustion chamber entirely. In this situation, you may see wet oil on the belly of the aircraft and in the exhaust pipe, but with much less soot by-product. The spark plugs on the other hand may appear perfectly normal. This is where the preflight procedure of running your finger into the exhaust pipe and visually checking the combustion byproducts can be extraordinarily valuable.
Lead Fouled Spark Plug: Lead fouling is characterized by hard, dark, gray, cinder-like globules. Observing some of the characteristics associated with the use of fuel containing tetra-ethyl-lead would be common on an aircraft utilizing mostly 100LL (avgas). This is the primary reason for the spark plug change interval on the Rotax 912 to change from 200 hours operating time to 100 hours. The Rotax 912 does a fairly efficient job of scavenging the tetra-ethyl-lead under normal operating conditions. And even using 100LL exclusively would not normally require maintenance intervention in between the 100 hour plug change interval. Observing an extraordinary amount of lead build up is normally associated with poor fuel vaporization associated with carburetors needing overhaul or repair.
Hot Spark Plug: (Figure: 6) A white clean spark plug free from any color is your most prevalent indication that the cylinder is running hot. Although the Rotax 9 series engines operate normally in an aggressively efficient manner and may appear lighter in color than what you would expect to see on the Rotax two-stroke engine, there will always be some color displayed on the ceramic insulator. In extreme circumstances when a spark plug overheats, deposits that have accumulated on the insulator tip may melt and give the spark plug a white glazed or glossy appearance. This is often accompanied with speckles of what appears like melted glass and it is common to observe melting of the center and ground electrodes. These symptoms are normally associated with conditions of lean mixture or advanced ignition timing.
Once again, it’s very important that you use this as an educational opportunity to understand some of the principles associated with reading your spark plugs. Understanding that every engine has slightly different operating characteristics and not all plugs will look the same under normal operating conditions. Once you become familiar the process of reading the spark plugs, you will realize what an invaluable resource this is in determining the condition of your engine. I’m amazed that we can have as many in-flight engine related problems as we have in aviation. These engines continuously shout and waved their arms letting us know that there are problems. When you’re reading a spark plug it’s telling you a tremendous amount of information about the condition of your engine. The engine will help you fix her, if you’re simply willing to listen. But as your wife keeps telling you. “you never listen.”