R912 CARB

What position is the needle clip?

Plugs often get black during idle.  When you did the most recent carb balance, did you check the idle-mixture setting?

Dry black soot can be normal from idling. Either run the engine up at high rpm's on the ground or go fly, but shut down immediately after with no long taxi or idle time. If you play with things your not sure about you may cause issue you won't like or cost money.

Mkae sure your plugs aren't gapped too wide which might contribute to the soot at  low rpms.

Rob may be onto something too. If someone was in the barbs make sure the needle clip is in the #3 clip postion.

thank-you for your prompt response,

My clips are in the standard position (2 it seems to me). Indeed, I have a lot of taxiing after landing. We're going to look at the idle screw. If I squeeze is it richer or less richer? I have a doubt...

Sincerely,

JSB

Clockwise is leaner.  The standard procedure/position is to turn fully in (clockwise) then back it out (counterclockwise) one and a half turns.  

The last time I checked mine, the screw on my 2/4 carb had somehow backed out to two full turns.