Running rich and bl...
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Running rich and black oil.

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Joined: 4 years ago
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I have attempted to tune the carburetor in May of 2021 and had relatively good success with the idle at the expense of running very rich. It was too lean as per the EGT temps and spark plug color. I made changes to the jets to richen, and it resulted in idling rough and even cut out on several occasions, so I settled on a compromise of running it rich.

This was the thread that Jabiru support was very helpful with at the time   

I have noticed though after my recent oil change and when checking the oil level, it is black, and I mean seriously black! I read somewhere on the forum that black oil, in particular if found when removing the valve covers (I just performed a tappet adjustment) can be a result of overheating. The CHT temps at crusie are 200 - 220 F with the exception of cylinder #2 which is running closer to 300 F. The EGT's on all others are in the 1275 F range with the exception of #2 which is closer to 1350 F. Crusie setting for the propeller is 2950- 3000 RPM. I believe the black oil is due to running very rich - every time I remove the spark plugs for inspection, they are very sooty along with the exhaust pipes. It's been running this way for almost 100 hours. The odd thing is that I noticed the darker oil in colder weather, it's winter here in the northeast USA and colder weather should produce a leaner mixture. 

Can you weigh in with your opinion, experience, suggestions? I do plan on attempting to tune the carburetor leaner at this point. 



Alex R.

Member Admin
Joined: 4 years ago
Posts: 190

Hello Alex,

If it running rich it is possible for the soot to get into the oil. Much like what happens with a diesel engine.  

You cruise numbers seem pretty good.  What is your hourly fuel burn? 

We have had some problems with the idle on some well used Bing carburettors.  Not sure if it due to some blockage some where in the system.  

But in cases where it does idle the idle jet has to be a large size to combat the hesitation or stopping when reving from idle. 

I have confused myself now.  Is the idle rich or lean?  

A rich idle will make the pipe and plugs sooty while you taxi back from flying.