What is happening is that you have a piston ported engine and there is a fuel/air charge that is blowing out the front of the carb. On the ground some is blown away by the prop blast, but a lot more is blown away in the air by the slipstream. You can feel and see the fuel/air charge if you hold your hand between the prop and carb blocking the prop blast. If you have it at idle and pop the throttle open while blocking the prop blast you'll see a "FOG" right over the carb opening. To test if this is the problem tape a air-dam in front of the carb opening, it can be a anything about 1" high by 2" perpendicular to the airflow, a servo or a playing card folded in half at 90° then cut the base so you can fold it to give it a little strength against the wind. I think you'll find this will cure your problem, then you can figure out how to make something nicer.