Several possible issues. One is ram air against the diaphragm opening, which does change with aircraft attitude. You don't want impact air, just static pressure. Another is fuel making puddles in the reed block. Both are common and require experimentation to correct. If the carb is spitting fuel you may have a reed that's bent or not seating correctly. Check and correct that first. A slim possibility is that you have an air leak at the carb isolator or reed block mount.

The diaphragm is the easiest to work with but may require several attempts to correct. One method is to run a tube from the diaphragm opening into a "dead air" space either inside the fuselage or inside a small box built out of balsa somewhere. It needs a pin hole to maintain correct static pressur. Another is to run a tube from the diaphragm opening to the front of the carb intake.

Good luck.