Black spark plugs

[yankee-flyer]

At 100 hours the plugs looked fine. At 200 hours all four front cylinder p[lugs were black-- not oily or fouled, just black. Rear 4 plugs were fine. Since they're fed by different carbs that shouldn't be the problem. EGT & CHT show no problems. Any thoughts???

Wayne 120241/143WM

 

[roger lee]

Plugs

Hi Wayne,

If you had black oil plugs that would be a problem.

The black dry velvet soot is not a problem. It is normal when you idle for a while. Although these things can add to the issue and can be caused by things like plug gaps. They should be .023-.027. The narrow gaps are for the colder winter weather. Wide gaps in cold weather can cause the soot. Thermal paste helps. That can equalize as much as 40+F between heads and plugs. Leaving plugs in too long before a change doesn't help.
Most have the dry black soot from long idles or idle rpms too low. If you run the engine up at 4000 rpm for 3-5 minutes and shut it off it would be all gone. You don't have it at higher rpms. This is just one of those things with our engine. The black plugs can even move around from one time to another and is different from one engine to another. The different length in the intake manifolds, one shorter and one longer on each side can also add to this.


It is normal so don't worry about it unless you need to correct one of the things I listed. If they are ever black oily and wet then we have another story.

 

[yankee-flyer]

THANKS Roger

I'll keep an eye on the plugs but I feel better now.

Wayne 120241/143WM

 

[roger lee]

I was concerned many years ago to until I talked to Rotax.

 

[bobnoffs]

this is just an observation i thought was interesting.........
annual last month and i replaced all the plugs in my jab 3300. i then pulled the plane out of the hangar and ran it for maybe 10 minutes at 1700-1900 rpm to warm it up for a leak down test. i have often heard how fast you can have deposits to look at, after taxi, etc. and was quite surprised to see these plugs were so clean they literally looked like new. i would have thought they would have already picked up their coating, consistant with the rpm.

 

[roger lee]

Hi Bob,

Air cooled engine vs water cooled, different lengths and styles of air intakes, different plugs, fuel mixtures, ect... will result in different results. I rarely ever see the dry soot on Jabiru engines. Even the leak down test has different results. You need to warm an air cooled engine for a good leak down test. The Rotax is so tight it can be done cold and still have the same results as warm.