Never assume that the little chinese man that assembled your engine just before lunch break actually set it correctly!

And did you manage to read the rest of my post?

Here is an extract from the OS 50 manual

BASIC POSITION OF MIXTURE CONTROL VALVE
(Mixture Control Screw)
As delivered, the Mixture Control Screw is positioned at the center as shown in the sketch.
Mixture gets lean when the Mixture Control Screw is turned right, while mixture gets rich when the Mixture
Control Screw is turned left.
With a model helicopter, adjustments vary with combined various factors such as climatic conditions, fuel, muffler, main rotor, weight of the model, gear ratio, etc.
Therefore, Mixture Control Screw position varies with each model and set- up, and it is normal
if the Mixture Control Screw position is off center.

From here incase you dont have a copy:

http://www.osengines.com/manuals/50sx-h-manual.pdf