Sounds like it is going lean. If richening it doesnt work, then take the high speed needle off and flush fuel through the carb, in case there is some crap in it. Also, is the tank set up properly. If its too low or too far back, it will cause it to be a b**** to tune. Another thing, check and replace (if needed or suspected) any fuel line.
Also, sometimes the O ring for the carb will be damaged. Check that out too. (and the bolt that holds the carb in)
Always try EVERYTHING before buying new.