This type of Chinese engines' problem is carb.My opinion is;some carbs are leaking fuel from bottom hole of spray bar instead of spray bar itself when throttle barrel is on idle position and engine vacuum is most.The pilot is feeling very rich of idle running and begin to leaning the adjustment by pushing the LSN into spray bar further.Then the leak of the spray bar bottom seems good for idling,pilot thinks adjustment is OK but in fact LSN is far reach inside of spray bar,practically nearly choke of feeding by spray bar.(all of those are in idling position).Then when giving full throttle the far reaching LSN is partly coming back and partly opening the spray bar to feed fuel.And seems good running but fuel feed is very lean in fact and very vulnerable against some fluctuations of fuel feed at carb inlet nipple.Then Pilot feels ready to take off and gives full throttle after some idling(which providing some reserve fuel inside crancase).The engine roars and pulls like a champion until plane gains some altitude but same time nose was looking up and tank feed to carb was in trouble mostly because of inertia and head of fuel.Then suddenly(just reserved crankcase fuel was spent at same time) spray bar feeding becomes lacking to feed the proper mixture.The main reason is still LSN was partly blocking the spray bar to deliver fuel though throttle was in full position.
Shortly this symptom can be easily seen by looking at carb on full throttle position if after your your good idle adjustment still LSN is in contact with spray bar mouth(at full throttle position).
If so, then I recommend you to disassamble the carb,make sure the spray bar o ring will not leak at all and reassamble.I hope I could explain.