912UL choke rough running

It does not sound like you have a problem.

The purpose of the choke is to temporarily deliver an overly rich mixture, if need when cold, to assist with starting the engine.

If it starts without the choke, don't use it.

If it runs rough after starting with the choke, Turn it OFF!

The choke is intended to produce a running engine, not necessarily a smooth running engine.

How cold is your Cold Start?  You may not need the choke at all unless the temperatures are below freezing.

I agree with Bill, have you tried just partial choke?

It is only an issue on the first cold start of the day when choke is absolutely required. It has always cold started fine on full choke (I don't recall ever using anything but full choke on a Rotax) and then one day it didn't and has stayed that way. As far as I'm aware nothing has been meddled with to affect it. When I say it runs rough I mean the engine is trying to break free of its mounts (not the normal rough run of an over rich mixture after start up) with massive side to side vibration. I tried it today and it does not start at all either with no choke or half choke as I expected. It needs full choke. It is cold and damp in the UK at the moment. 

After the first cold start no choke is required for further starts later in the day unless it has got really cold again. It starts and runs absolutely fine then with no choke at all. 

I am making the assumption that as all other carb operations appear fine apart from a cold cold start then there must be some issue developed with the choke circuit. The ignitions are working fine at the moment so I am tending to rule that out thus far. Is the choke operation more sensitive to air/ fuel blockages than the other circuits?

I am am completley puzzled! I am guessing that there is either too much or too little of something (air or fuel) either supplied commonly from both carbs, or singly from one. I am swaying towards an imbalance in mixture supply between the carbs as the rather violent vibration that is happening suggests that one side of the engine is firing much better than the other (if one side is actually firing at all). 

What channels could possibly get blocked in the choke circuit to cause this but not seemingly have much affect throughout the rest of the Rev range? It did feel like I was getting airflow through all the orifices with the air gun on both carbs. Could airflow seepage past poorly sealing o-rings have an effect although these didn't seem to look that bad?

Hmmm. Something has changed but I can't work out what!

Make sure the pistons slide easily and freely up and down. They are pulled up by vacuum only. You also mention the o-rings were in bad shape on the idle jest, that could mean that there are air leaks past the throttle shaft o-rings. Replace all rubber parts in the carbs. Check out these videos on the choke/enrichment circuit and theory of operation.

https://www.youtube.com/watch?v=hDsJH1Y9W_U 

https://www.youtube.com/watch?v=xWkZ-Acz1L0

Great video!

Funny you should say about the pistons. I was wondering if one piston may be sticking slightly open internally when the hand and foot throttles are both fully back at the idle stops which might override the choke circuit on one carb. Again I don't know what might have caused this but there is scuffing present on both pistons but much more evident on the port side carb. Odd this for an engine with only 500hrs. There is polishing of the needle evident on this carb also and when doing the carb balance (I have a carbmate) the balance tends to temporarily waver out and back into balance when the rpm is altered more rapidly through the low to mid-range suggesting to me either that something may be momentarily sticking/ releasing and/ or there is wear on the needle/ needle jet. There has been work done in the past on one carb more than the other as internal parts are different (eg. Black viton fuel valve tip vs red viton tip fuel valve). This puzzles me as to why one carb has been fully stripped in the past but the other hasn't. Makes me a little suspicious.

The one set of o-rings that I didnt sight was the butterfly valve o-rings as I can't get to them. I have said based on the state of some of the other o-rings that it might be time for a complete overhaul and a whole new set of o-rings and gaskets. I guess I am trying to elimate other possibles before doing this. Is there any reason why the idle mixture screw o-ring would be in a much worse condition seemingly than the rest of the rings in the carb (can't see the butterfly valve o-rings without fully dismantling it). 

Thanks for your input