Are you sure someone hasn't moved the e-clip up on the carb needle? Usually people that drill the muffler raise the clip to richen the mixture to make the bike run properly. It
does sound like you are running too rich.
Check this thread
http://www.gz250bike.com/viewtopic.php?t=617&start=0 and it explains how to raise the clip. You may want to check yours and reverse the procedure if it has been done.
