912 ULS Rough Running

I had the same issue, it drove me crazy. i found the problem, Vibration! one of my carbs was in slight conntact with the upper cowl.. I had to cut the upper cowl and make a bubble to make room for the upper part of the carb. I think in flight there is a slight movement of the entire engine. Kind of makes sense , everything is attached to rubber mounts. I hope this helps. If it does please let me know.

Thanks for your reply. No, I don't use a fuel funnel or strainer when putting fuel in the tanks. Not seeing anything nasty when I sump the tanks before flight. I am switching to 100LL to see how that goes.

Thanks for your reply! I don't have a fuel pressure gauge, or anything sophisticated instrumentation-wise. Only oil temp/pressure and a single analog CHT gauge for the #3 cylinder.

Here's what the MOGAS looked like after sitting out in a glass after about 25 minutes. It's clear in the tank, but when exposed to outside air it turns into something looking like grapefruit juice!

OMG! I can't believe what I'm looking at!  

You're a Colts fan? :)

Seriously, that gas ain't right.  How old is it, and where did it come from?  I'd drain all of that out of the aircraft, including the carb bowls, and check the filters before adding fresh MOGAS (from a different source!) or AVGAS.