West 50
We hear this comment a lot over here...
I too had problems with that at first.
Make sure you press he carb down HARD and then tighten the screw.
The O-ring under the carb MUST be compressed, or the engine WILL suck air past it.
If the carb is tight it will stop instantaneous on the ground and after about 1 or 2 seconds in the air, where the prop is driven by the wind...