Have you taken the carb apart? Cause when you open the throttle your not only allowing more fuel but more importantly more air. I had a motorcycle do this once and the idle jet was out and sitting in the tube. It was getting to much fuel and opening the throttle gave it air.
Good point... another issue that I have seen is dirty carbs. Remove the carb and make sure that you clean it really good then make sure that the jets are clean...
Keep us posted...
