Re: how to adjust
There is no mixture adjustment screw.
You can move the circlip position on the needle to change the mixture though. lower is leaner and up is richer.
The one screw on the carb is for adjusting the idle rpm.
It's a 2-stroke motor, the back oily residue is caused from the unburnt oil/fuel mixture.
That is exacerbated if you mix in too much fuel. Try 24:1 for the first gallon, then got to 36:1-42:1 after it's broken in.