The two airflows: 1 from the prop wash, and the other from the plane moving through the air, causes too much air across the face of the intake of the carb.. It acts like a siphon (it sucks the air out of the intake instead of pushing air into it). The carb then goes rich and chokes on too much fuel. If the intake is exposed directly to airflow, you might have this problem, but if you can cover the intake with the cowl, it should stop. It is just too much air over the carb. If it runs ok on the ground with just the prop wash, then it is as I have suggested. Since I could not get the intake under the cowl, I had to buy a 3W velocity stack which has worked out very well for me. The 3W distributer here told me that I could make one from sheet metal that would direct air into the intake but I just bought their stack and have no more trouble.I hope this has been of some value.
I could not email to Christian so this is for him in Romania.