Looking for plug read / mixture assessment

Hi Chris,

I have had the same experience you have described with my 912UL on a Kitfox IV. Plugs look just like yours every time except after approx. 5 minutes at 5000 rpm with throttle back and shutdown. The rear plugs remained with their light color and the front plugs lightened up. I tried the needle change by 1 notch but the rear plugs came out a little too white and I decided it was not a good idea if I could not back it up with egts. After installing egt probes on the rear cylinders, I find that I am running at the recommended temps on the monitored cylinders so I will not be changing the needle position away from recommended.

I have checked floats, cleaned, rebuilt and balanced carbs to exhaustion. Experience from a previous 912UL on a trike also produced sooty plugs but it never seemed to have an ill affect on the engine.

After some internet searching I find that this is not such an unusual issue and have not found any definitive solution.

Good luck. Please follow up here if you solve your particular mystery.

Charles Snyder

"...also produced sooty plugs but it never seemed to have an ill affect on the engine."

I wouldn't care--or even know--what my plugs looked like, if it ran smooth in the first place.  I was happy to pay a mechanic, until I realized the only thing that was changing was my bank balance.  If I was ever going to trust this thing, I had to get involved.

Hi Chris,

It seems to me (not the sharpest tool in the shed) that the Rotax carburetted 9 range, have pretty simple fuel & spark delivery mechanisms. Simplicity tends to deliver reliability (which the 9's have in bucket loads) BUT there are compromises. 

The igniting system is not very much more sophisticated than that found in your average lawn mower/chainsaw/etc  - there is no proper ignition advance retard, like you would find in your (older) car. If you have "soft start" ignition there is a basic start ignition timing but as the revs climb you wont get optimum spark until (I guess) over 4500 RPM. What has this got to do with "sooty plugs" ? This simple fixed ignition timing, optimised for cruise/climb power/RPM,  will deliver sub optimal fuel burn at low RPM's, resulting in soot deposits inside your combustion chambers. Easily fixed - go flying. Most if not all of the soot will burn off during climb out & cruise. Unfortunately it will return at decent (idle) power and taxying.

The carburettors are also a relativly simple mechanism for delivering a fuel air mixture  - they can not deliver the precise fuel/air mixture of an electronic fuel injection (EFI) system. Like the ignition system there will be inefficiencies at certain RPM's and density altitudes that may lead to slightly over/under optimum ratios. You can get much better fuel efficiency but it will cost you big time - its called a Rotax 912 IS

UPDATE for future readers:  One year after I started this topic, I received word that the quoted oil report (from post #4) was actually the result of mixed-up samples, and has been corrected.  It looks normal now.