The following is what I've done. What else should I do?
Finally started my Rotax 912 for the first time last week Ran smoothly for all of 30 seconds. Then really roughly. Then shut it off and called it a day because it was getting late. Had time to think on it, and thought maybe I hadn't tightened up all the cables and doo dads. The throttles are in the swinging arm rests so there's a lot going on (it is what it is, let's not debate what the best design is).
Anyway, I adjusted everything. Hoping that it was just running unbalanced. Didn't make a difference. Next guess was a blocked jet in a carbie or something. Because I always half arse everything , I found I could loosen the carbies, and remove the bowl without having to undo the cables and hoses.
At first glance the bowl looked quite clean, but when I looked closer I could see that petrol 'varnish' that can occur over time. Not heaps, but it could cause a problem I guess. So I carefully scrapped all that off and rinsed with turps.
I also took out the (pilot?) jet. I couldn't tell at first if it was blocked or not. But after a bit of blowing I could see all the way through. Could have been blocked, or just fuel in there blocking the light. I poked a copper wire through to make sure.
The jet in the bowl (choke? Idle?) was clearly fine and I could blow through that.
Once assembled again I still had difficulty starting it. When it did start, it ran smoothly maybe be for a second or so and then roughly again.
I did a few times in case something got sucked through but I'm guessing that wouldn't be likely to happen. Because in total it was enough time for the cylinders to warm a little, I noted that the right cylinders were cooler than left, which seems to me to indicate the problem is the right carbie.
Oil pressure was good.
Spark plugs looked ok.