I had a similar problem with my Brillelli 26cc on the front of a Giles 202. It ran good on the ground but once it broke ground and got in the air it would spit and sputter like it was flooding out and if I cut it to idle it would die. To make a long story short, it ended up being the air flow coming around the cowl to the carb. To fix the problem I made an air induction out of a 90 degree brass elbow that came off the carb and faced towards the prop. Problem solved.