Same issue with a Yak with a 3W 55I Us Edition with a side carb. Could set it perfectly on the ground but when it got moving on the takeoff run it would go rich. I had to block airflow over the carb throat. I learned this from Gearhart at Aircraft International (3W Distributor Supreme). He described this as blowing across the top of a pop bottle. It causes a reduction in pressure over the carb throat. Since the carb doesnt sense pressure there it puts in the fuel it thinks is needed where it is sensing air pressure hence a rich mixture.