Maybe you could check the reed valve condition,maybe the valve has a broken part and not seating properly.Did you try it w/o air filter,if you feel a difference?Check also if the exhaust is not blocked partially.
I have three carbs (C1, C2, C3) and two reed valve assemblies. I tried the C3 with the other reed valve and couldn't get it to run well, so I tried the C2 with the other reed valve and it seems to be running pretty well. I'll try to get to the wood lot this weekend and give a good workout.
Thanks for your help.