Lesath,
What you are seeing is completely normal. On the engines we use has a lubricant in the fuel (either castor oil or a synthetic lubricant) and this lubricant does not burn in the combustion process. This is what you are seeing coming out of your engine. While still 2-stroke engines our motors differ from a "weed wacker" in the fact that on the weed wacker engine the oil usually burns off in the combustion process.

Just wipe your plane off when you are done flying. Everything is normal.

Hope this helps

Ken