Now here is an important point and I see many mechanics and/or owners get it wrong, especially those who have not attended a Rotax school.
DO NOT APPLY ANTI SEIZE to the plug threads!
You should be using a thermal conducting paste which can be purchased from any Rotax distributor. Other aircraft engines do use anti-seize, but Rotax does not. As a matter of fact, I have many GA owners now using this thermal conducting paste over anti-seize. Anti-Seize did its tour of duty, but the new technology dictates that it’s time to move on. The paste will help with any galling issues that sometimes happen when you install a dry plug. Do not install the plugs dry (except for the 2 strokes). This is a silicone based white paste which is also used in the electrical contact industry. Apply the heat conducting paste to the top 2/3 of the plug. Do not get this on the electrode because it will not burn off and cause the plug to misfire. When applying the paste there is no need to glob it on. In this case just enough trumps too much. I use my small finger with just a dab on the tip and rotate the plug to spread it on the threads evenly.
The plugs now can be installed and torqued to 177 in/lbs (14.7 ft/lbs). Now this is a point, but one that is too often overlooked, so stay focused. It’s all too common to find loose plugs simply because someone got sidetracked and forgot to tighten them.
The following picture shows what they may look like. The top plug was torqued in place and the bottom one was loose which caused bypass gases and fuel blowing past the threads.
Make sure you push the spark plug cap back in place correctly and hear that unmistakable ratcheting sound as it slides down over the terminal end.
Side note: If a plug starts to bind or become hard to thread, STOP. Remove the plug and run a thread chase down the threads to clean them out. Forcing a plug when it is hard to turn will only damage the plug threads in the head. They make an inexpensive, special tool just for this and you’d be wise to have one in your tool kit.
What is my plug telling me?
Much will be learned about your Rotax engine condition, as well as how it is running, 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 inspecting your spark plugs.
Look at the spark plug porcelain for the color and condition. Plug colors can tell you how the engine is running and obviously, that can be very important.
Plugs change color and the different colors can indicate such things as engine overheating or if the engine is worn. The color can be ascertained by looking at the porcelain insulator and then by comparing to the plugs shown in the next few pictures.
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A tan colored plug means that the engine is running normally 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. This plug shows normal wear in the center electrode. A new plug will have square edges that help the plug fire more efficiently. |
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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. |
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This plug shows what can happen when something hits the spark plug from inside the engine. This problem must be repaired before running the engine further. Make sure the spark plug is the correct length for the engine. |
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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. |
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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: Incorrect spark plug (heat range too high). Low octane fuel. Timing is not set properly. Cooling problems, (dirty cylinder fins; no, or low, water on a water-cooled engine; 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. |
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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 by using a fuel additive. This condition will cause a misfire. This can be also caused by changing oils in midstream. |
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This plug is oiled fouled, caused by poor oil control. |
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Pre-ignition will usually appear to have 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. |
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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. |
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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. |
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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 causes for a rich air/fuel mixture are: A dirty air filter, The air mixture screw or carburetor needs adjusting, The choke is sticking, or the carburetor float height is out of adjustment or float is sticking open. |
Here is a good reference:
http://www.ngksparkplugs.com/tech_support/spark_plugs/faqs/faqread.asp
Understanding the numbers on my spark plug?
Related Videos: Preventing Detonation
Comments
The plug for the 912ULS is DCPR8E. No "S" on the end.
thanks
Gérard
It is .023 - .027 inches or .6mm - .7mm
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