Personally, I usually tune 3W's and DA's with a 15 lbs sledge hammer

Seriously, you are running into a pressure problem inside the cowling, making the pressure regulator inside the carburetor change position and affecting the mixture once you get a lot of air passing through the cowl (when airborne). Best bet is to isolate the diaphragm assembly with a breather tube into the fuselage, or by using a block-off plate or one of the 3W intake horns. All three methods work well. It's also a good idea to prevent any direct airflow against the carburetor by using an isolator plate to protect it from the incoming air blast. I do this by keeping the bottom of the cowl closed, and the cooling duct work keeps the air coming in from the front inlets from getting down to the carb and causing a problem.