Is my engine running lean?

Hey guys.. I am attaching a picture of a plug from my number 3 cyl on top .. it is indicitive of all 4 plugs in cyl's 3&4 .. This was an old plug I used to prove a point and it is after flying for approx 2hrs. The plugs in cyl 1&2 (all 4) are a dark brown color.. I did taxi back to hangar before shutting down. This is a 'new' rebuilt engine I purchased from a 'reputable' Rotax repair shop. it currently has 70hrs on it. (long list of issues that I have gone through to this point but I will save that from this post).

I do notice a mag drop that seems to be quite a bit worse as the weather cools off here (seems to run quite rough on my 'left' mag).. And it seems to be running rougher as the weather cools too. I have check a few basics with removing the float bowl and checking floats. I also removed diaphram and confirmed clip is in position 3 which is spec for this engine. (912 80HP) , and the obvious balance carbs about 10 times.

Also I have changed the plugs, caps, cut wires to ensure good fit, changed one coil as that is what the mag drop seemed to follow (cyl 3&4 top plugs seem to run the worst if that makes sense).. I seem to have altered this post as it went but wanted to try and provide all info I can.