912 ULS Rough Running

I have a 912 ULS installed on my 2006 Evector SportStar that has been giving me problems. The trouble initially started back in the fall with a momentary rough running engine when in a full power climb. Engine roughness lasted a second or two, but was bad enough that I declared an emergency and landed. Rotax-qualed mechanic found the original equipment carb bowl gasket (cork) were bad and replaced with new synthetic ones. A couple of flights later, the engine would barely stay running at idle on the ground prior to takeoff. Carb bowls were dropped again and small flecks of debris were found, along with a small amount of water. Jets were cleaned and a successful run-up ensued. A couple of flights later engine was running rough at idle on0deck again. This time the carb floats were found to be bad, with small black flecks easily coming off if you dragged your fingernail across it. All four floats were replaced, the jets cleaned,  and the carbs rebalanced. Airplane continued to experience brief moments of engine roughness that could not be reproduced on the ground. I would land, do an engine runup, and everything would be normal. Very frustrating. Airplane then went down for maintenance while the original equipment brake system was replaced with a new MATCO system.

During the first flight after the brake replacement, as I was preforming touch-and-goes and at 350'AGL on the upwind at full power, the engine started running so rough that I declared and emergency, performed a180, and landed wrong-way back on the runway I had just taken off of. Upon clearing the runway, I did a full engine runup check - and everything was normal. Called another ROTAX mechanic: carb bowls were dropped (no debris), carb floats were weighed (all were normal within limits), and balanced the carbs. He also suggested that I not run MOGAS, saying that "I've got three guys with the same engine running MOGAS and you're all having the same issues". Winter blend gasoline appaently has all kinds of additives and other crap in it. To illustrate, He had pull some gas out of the wing tank and put it in a clear glass. The gas went into the glass nice and clear, but after 25 minutes of sitting the MOGAS looked like grapefruit juice: cloudy with a slug of brown residue rolling around the bottom of the glass (see attached picture).

We changed the fuel filter (not too bad), drained the MOGAS and replaced it with 100LL. Did ground run-ups on two seperated days before I actually got airborne again. Did a functional check flight over the field up to 7500" with no issues. Problem solved! Flew it again a week later and on climbout after the initial takeoff, the engine stumbled again. Not nearly as bad and only for half a second, but enough that I just turned downwind and landed. 

I am very frustrated to say the least. My background: Retired USN and retired Southwest Airlines. Been flying this airplane since 2015 and have had no issues with the engine up until now. Engine rubber was replaced on schedule in 2017. Airplane resides in Sand Diego area and has always been hangared and meticulously maintained. Would really like to enjoy flying again to exotic destinations beyound a 3NM radius from home plate.

Any suggestions you all might have will be greatly appreciated!

Warmest regards,

Derek Martin