I had a PA50-II with the same problem. Clean the air filter with carb cleaner and then dip it in motor oil (10W30 is fine). Wring it out and put it back.
Clean the bejesus out of the carburetor. It's the main jet that's giving you problems. You have to get it out and clean it with a wire.
When you look into the carb from where the bowl goes, you have one jet, that can be easily unscrewed. The jet that's probably giving you problems is the one behind that one. Get it out and clean it until it looks brand new.
If you need help getting the carb off (it's a procedure and a half on a PA-50) let me know and I'll write a quick walk-through.
You might as well put a new fuel filter on the moped while you're at it. Since you practically have to take the engine off to get the carb out, you don't want to risk a piece of crud soiling your carb five seconds after you put it back on. Buy any cheap, small, inline fuel filter at an auto parts store.
Have you modified your variator on your PA-50? I know an easy trick that you can do that'll give you an extra 5MPH.