Probably running the engine too lean.

When tuning the engine on the ground, you should use the needle to get the maximum RPM without going too lean, AND THEN BACK IT OFF A CLICK OR TWO. In the air, the prop will gain RPM because of the airspeed of the plane (the prop does less work in the air than when the plane is stationary on the ground). Further, when going vertical, the suction produced by the carb that pulls the fuel into the engine has to overcome gravity, as well as G'Forces to get the fuel. Thus, you engine over-leans and starts to die out.

Wou will not have this problem at half-throttle.

gus