Outboard carburetion is my bane as well..as handy as I am I havent mastered it yet..and end up paying the money
to have them tinkered with after I have exhausted my patents.
In my outboard experiance electrical
isues genrally show up with a dead cylinder period or start acting up once hot...my guess is you are corect and its an air fuel
mix at idle...I hate that..most likely junk in a tiny fuel/air Passage
I have an 18 hp Johnson 2 stroke
that some mice decided to build a nest in over last winter sitting on the floor of my shop..I did not notice it untill 200 miles from home on a fishing
trip with my buddy's..it just died at full throtal running across the lake...I popped the cover and there was this huge mess of pink fiberglass insulation
the little buggers had pilfered out of my shops walls.
Well I sucked some of that crap in the carburetor and I never did get it to Idle well that trip...back home I got busy with life and put it off long enough to completely forget about it until this year when it was to late do do anything about it..so I just loaded it up sort of disgusted with myself and off we went on our yearly trip to the same lake...not a big deal really we only use the gas moters to usually fly back in at the end of the day or to get off the lake if our trolling batteries die or the wind
over powers them....well lo and be hold it ran like a champ the whole trip after a couple of the same tweaks to the crab I had tried the year befor to no avail ...go figure.
Moral to the storie... you could just Keep running it and it may clear itself out.
As far as the dammage you have done to the flange ..if it causes a vacuum leak you have got to fix that somehow.
The soot is just from the wrong fuel/air mix..It will disappear when all things are corect.