My 912 ULS is 4.3 years old. It sat for 3 years before being put into service. Rotax long term storage procedures were followed. It has 188 hours since going into service and has performed without any problems until the last attempted flight. The start was normal including a run up at 4000 RPM with mag drops of 100 RPM each side. Seconds after going to full throttle for take off the engine started running very roughly with a large RPM drop and loss of power to the point that it stalled. However the stall may have been caused by my panicked pull back on the throttle to abort the take off. I didn't roll more than a couple hundred feet on asphalt. I have a facet fuel pump backing up the mechanical fuel pump and it is always on for take off and landing. After checking everything that I could think of I did a restart on the other gas tank. This time I was lined up for take off but not planning on going anywhere. I got exactly the same performance. Everything is fine until going over 5000 RPM. The gas is fresh 94 octane mogas (Canada). I tried with the electric fuel pump on and off. After a dozen tries and cowl removal and trying I gave up for the day.

The next day I replaced the spark plugs and again everything was fine until going over 5000 RPM. If I throttle back it will sometimes clear itself at a lower RPM without stalling but this is not immediate taking perhaps 30 seconds before it runs smoothly.

I have attached a picture of two plugs, one removed at 100 hours and the second removed since the problem developed at 188 hours. The bottom left plug with the beige deposits is the 100 hour plug while the upper right plug with the black deposits is the plug removed after a dozen tries as mentioned above. All 16 plugs are showing the same characteristics, eight beige (100 hours) and eight black (188 hours).

The black deposit makes me think (in my infinite ignorance) that the engine is flooding. I checked the chokes several times and they are indeed going to the off position.

And a special thanks to my engine for quitting at 0 feet instead of 300 feet. My landing options are trees or water.
4bottomplugs.jpg (You do not have access to download this file.)
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