Soot? No it is carbon mixed with oil.
Not enough oil. You mentioned that you are running a 40:1 oil mix - it should be 30:1
See the Tech details here;
http://www.dle-engines.com/dleg0031.html
Regardless, it sounds to me like you need to tune the engine as it is not burning the fuel properly which is causing 'Soot' which is sticking to your plane compliments of the oil mix.
Lean the engine a bit and confirm the engine speeds with a tach.
Happy Landings!