912UL choke rough running

If it will not start without choke and tries to commit suicide with the choke; I agree that One side is probably dead and the working side is causing a gross power imbalance.

The Rotax "Choke" is a misnomer.  It is really an Enricher circuit.

Where common chokes "Choke " Off the air to the engine causing a rich mixture; The Rotax Choke does not affect the airflow but opens an additional fuel path to "Enrichen" the mixture.

The symptoms are more in line with one side operating normally and the Bad Side Overdoing it, as it runs normally after removing the choke.

The fault scenario is more in line with a Rich/Flooded condition resolving to Normal/Normal than a Rich/Normal resolving to Normal/Normal.

I would pull the chokes off and give them a close inspection for debris.

You should be able to tell which side is not firing by watching the EGTs during the Start.

Or just attempt a start with the choke ON and shut it down after a second or two and feel the exhaust pipes.

The cold ones are the dead ones.

I love that term 'tries to commit suicide', I'm going to use that one in future ?

Good of idea on the EGT's/ feeling the exhausts I'll try that one tomorrow. 

I did remove the chokes, cleaned them up and blew them through with compressed air. Now I've watched that video I am wondering if something larger might be lurking in between the two plates of the choke.

One thing that I found puzzling was that the fuel that ends up in the float bowls of each carb seems to be different in colour even though it is coming from the same source. The fuel in the SB float bowl is a dirtier brownish tint colour whereas the fuel in the PT float bowl is much clearer. Suggests that the fuel is being discoloured by something en route to one carb but not the other. Even after a good clean of the carbs the fuel is still discoloured in comparison but not quite as much. I have never seen this before on any aircraft.

If it were a larger blockage in the fuel line or the carb it would surely affect operation over the rest of the Rev range but it's not. 

I was also thinking too much fuel on one side given the odd explosion it makes every so often like it is igniting excess fuel at the wrong time and place 

Got some things to try at least thanks

Be sure that both enrichener ("choke") circuit cables activate the circuits' levers evenly.  When the "choke" is OFF, each of the cables should be pushing the actuator lever fully against it's OFF stop -- a bit of excess cable bulge here helps to confirm this.  In addition, the actuator springs should be in good shape and working properly to fully assist in turning OFF the circuits.  When the "choke" is ON, each cable should be pulling the actuator fully against it's ON stop.  In between these two limits the actuator levers should basically have very similar coordinated movement.

Considering the float bowls.

Also, After watching the videos, I was not aware that it also drew in air.

The float bowls have a small side chamber that the choke draws its fuel from.

The fuel is drawn from the very bottom of the bowls which is also where all the junk tends to settle.

If the small hole in the bottom of that chamber is blocked; The choke will not draw fuel but will still admit additional air resulting in an even leaner mixture than normal.

Your problem is probably something simple, (They always are After you find them) and this fits the bill!

Great rebuild video.

https://www.youtube.com/watch?v=MpBA6wimjLc