Remove the plug and crank the engine over fast and long so as to dry out the crankcase of any fuel. Do this with the throttle wide open, and choke OFF. Then replace the plug and try starting normally.
If the petcock leaks at all it can cause the problem. The only way I know to fix that particular petcock is to replace it. It has a flat, round rubbery (maybe neoprene?) disk in it with four small holes. I don't think you can find a replacement for that part. The entire petcock can be found. I found a new one on eBay but I think some of the moped dealers have them also.