black spark plugs, probably oil burning

! + 2 are usually the colder cylinders as they get the freshest air and collect soot on the taxi after landing.

If they keep looking the same and not getting worse, you are just cleaning them up in flight and sooting them up on the ground.

If they progressively get worst with time and you notice missing Oil, then worry.

Many Rotax engines will have black sooty plugs at times from idling at the lower rpms. That goes away at cruise rpms. That mixture can be adjusted slightly, but it usually isn’t that big a deal. The carbs should always be synced at idle and the upper rpms (i.e 3500), this is very important and don’t neglect it. I sync all 912 engines every 100 hrs. If you have a ground adjustable prop it should be set to get at least 5500-5600 rpm WOT flat and level and at your average cruise altitude. Then you should be cruising at 5000+ rpm and not the mid 4000’s. All this comes into play. The length of service time on the plugs and the gap makes a difference too. The plug gap range is .023 to .027. Use the smaller gap for the cold winter months and the wider gap for the summer months. Don’t leave plugs in service too long. I change mine at 75 hours and others at 100 hrs., but I wouldn’t go over 100 hrs. If you can’t get up to operating temps then use a piece of 2" wide aluminum tape across the coolant and or oil radiator. If that isn’t enough because it’s real cold add more until the temps come up. Unless there is another problem at play most of the time these few little items can make a difference.

Assuming you have a carb running 1/2 and another running 3/4, I’d say they’re not synced.

Huh? I think it's usual to have one carb on 1/3 and the other on 2/4, no?

Ohhhhh, writing mistake.

Looking at those plugs it appears to be a mixture problem, not oil. If the engine appears to be running well then I would very carefully clean out the very small drill holes in the carbs down stream of the butterfly valve. Blockages here can give rise to poorly atomised fuel that has a tendancy of making the front cylinders rich. Be very careful cleaning the idle circuit, use carb cleaner and compressed air.