912ULS intermittently loses RPM, vibrates strongly

Andrew Dunning wrote:

On a standard Rotax setup (sprung open) an intermittent sticking throttle cable will show the symptoms that you have described.

The out of balance cause the carbs to flood creating a over rich condition (black smoke), pulling back the throttle evens out the carbs, stopping the vibration.

I had not considered a sticking throttle cable, but I should have. Still, I don't see how the engine could be brought to full static RPM (4800 - reached 20-25 seconds before the rough running began) if one of the throttle cables was sticking. Also, the smoke was not continuous, but two instances (I called them "puffs") of about a half second each towards the end of the 30 second event.

After seeing the video of the puff of smoke, my mechanic (Jim) consulted the Lockwood people, and all agreed a sticking exhaust valve was the most likely cause of the RPM loss and vibration. Jim did the valves on all the heads, and the engine runs smoother and about 300 rpm faster at full throttle on the ground. The #1 cylinder exhaust valve was the culprit, with thick black, almost "gooey' deposit about 1/2" long on the valve stem from the valve head up. The valve was tight in the guide, and the guide was difficult to ream out, but both valve and guide were in good condition, once the deposit was removed. The other exhaust valves were in good condition, but perhaps with more deposit on them than normal.

I did not get a picture of the #1 valve, but the attached image shows the worst of the other valves. On #1 the entire deposit was as black and thicker the black ring on the pictured valve.

Jim was not sure what caused the heavy deposit on the #1 exhaust valve, but offered several suggestions:

• The extended idling I do while soaring (gliding) might not be keeping the valve hot enough to prevent deposits from forming. I estimate I've idled in flight about 200 hours out of the total 600 hours of engine time.
• Possibly, lead from using 100LL might cause a sticking valve, but most of my flying is with E0/91 AKI fuel, so that isn't likely.
• He suggested I test the E0 mogas I've been using; I did so, and it tested good (no alcohol) so it's probably not the fuel
• The only time he's seen such a thick, almost gooey deposit, was in plane with tanks sealed with the wrong product for fuel with ethanol. Not likely my problem, as none of the other 50+ planes have had any problems.

Of course, I want to avoid this problem in the future. One way is to do my soaring with the engine off, propeller feathered, as was intended by the designer! There can be difficulties with that approach, but I think I can cut the engine idling time in half with only a bit more effort while soaring. Another possibility is some additive that reduces the build-up of carbon on exhaust valves, but I don't know what it is. Any ideas?

Thanks to those that responded with ideas about the problem. Even though none of them solved the problem, they did get me and others think more carefully about the situation.