how can i fix my crrc 45cc aeroplane engine?

hi, i have a crrc 45cc model aeroplane engine fitted with a walbro carb No WT 734.the engine starts with no problem but will not accelerate up from low to mid range without cutting out, i have had it back to the U.K. distributors three times but with no success, they keep on saying that it is fine.i have fitted a new cdi unit thinking that maybe the timing was wrong, i have replaced the parts in the carb and read several issues on tuning walbro carbs, i have tried on several occasions to tune the carb but i cannot sort the problem out, can you please try and help me with this matter, thank you dave.