Iceland Winter

I've certainly operated my Rotax 912 in the UK below 7°C with the needles in the default position without any problem. You could check that the circlip is in the correct position but I doubt that it will be wrong if they are new.

Hello Ami

A mag drop of 4 to 500 suggests a lean burn. (should be less than 300)  I would try to lift (enrich) the mixture one clip position.  I would have to assume you have looked at all the other normal things like carb balance? Also check for any signs that the jetting might have varnish from old fuel in the carbs.  Look for discoloration of the jet parts in the float bowl.  If there is dried fuel on these parts it may rob some fuel flow and give you leaner burns.  The engine is set to run leanest at mid range for best fuel economy.  At full throttle it will be rich for takeoff power.  

Remember that cooler air is more dense and therefore needs more fuel. Try a test run and when it starts to shake activate your choke to see if this helps.  If it does for sure it is lean. 

Cheers

I fully agree with RW on colder air makes an engine run leaner and at times increasing the richness of the carbs can help, but.......

Rule out other causes too especially since it only seems to affect these three engines.

7C is only 45F and shouldn't be giving you any trouble. During the winter months here I fly at times from 20F - 35F without any issues at the normal #3 clip position on the needle and on some occasions its been in the single digits at high altitude without issues. Others in the northern US fly in even colder temps. Many around the world do also. 

If the other 50 engines that are there don't have issues and they all have the clip in the #3 position on the needle then I'd want to rule out other causes before I just moved the clip to the #4 position. Check plugs, do a carb sync and since these three are the only ones with this issue and are brand new check wiring. 

Bottom line: What's different from these three engines vs the other 50?

Got stranded at Reykjavik airport in Iceland in January for several days once long ago when our airplane broke.  We discovered adding alcohol to our internal combustion stomachs at the airport bar enriched our physiological mixture sufficiently to smooth out our rough running attitudes.

Did the burping procedure, too?