Hmmm...I wonder if it's flooding out.
You might be getting some weird suction effect around your carb venturi once you're in the air, causing a super rich condition at idle. I'd say a velocity stack would be worth a try, but that motor has a rear mounted carb doesn't it ?
Usually if it runs great on the ground and acts goofy in the air, it's an air/fuel metering problem revolving around too little or too much air pressure around the carb venturi.