Does it still have the exhaust restrictor in place? It will definitely run better without it.
As I understand it, all PW50s are jetted hopelessly fat from the factory, making an already weak engine even weaker. You might want to try a smaller main jet in it.
You can also disable the oil pump and run it on premix. The oil pumps seem to put in about five times as much oil as the engine needs, which turns it into a wonderful mosquito fogger but makes it run poorly. To do this, pull the oil pump, remove the plastic gear on the end of it, and reinstall the pump. On the one I did, I also removed the oil lines and tank and plugged all the oil line ports to keep dirt out of the pump and keep from screwing up the carb. I filled it with the same 32:1 premix I use in my CR250 and the little bugger ran pretty well.