It's a stock 2005 Streetmate with 1750 miles on it. It ran perfectly for two weeks, and I put 550 miles on it myself. Then, it slowed down and stopped all together. Before figuring out that the muffler was the problem, I changed the gas filter, spark plug, transmission fluid, checked the jets, and cleaned the air filter with water and dish soap. The gas cap no longer has an o-ring and just sits loose. All of that didn't seem to be the problem. I pulled off the muffler and it started up and moved just fine. I tried setting the muffler in a camp fire to clean it out. I had drained the oil tank when I was figuring out what was wrong with the moped, so I premixed about a quarter of a gallon to give the oil pump time to start circulating oil again. I put the muffler back on, and it ran for 20 minutes, then stopped moving. I walked it over to a gas station. I figured then that I had wasted my time trying to clean out the gunked up muffler, and I just needed a new one. The oil lines looked like they were circulating oil again, so I topped off the gas tank to try to dilute the oil I had mixed and poured in there. I pulled the muffler off, put it in my backpack and rode it the 20 minutes back to my house. It was loud, but ran fine. I bought and installed a new stock muffler and exhaust gasket. It's now stalling constantly. It'll go for awhile if I ride it with the choke switched up, but stalls. On the 20 minute ride to work yesterday, it stalled about seven times.
I don't know if it now has too much oil in it, with what I had in the tank? I've been moving the knob around on the carb to see if that changes anything. I think I smell something like hot oil. It's hard to say.
The strangeness in the shifting gears when it reaches about 15 miles an hour, is just more apparent now, I think, than when I was riding it for those first two weeks, which seems now like a long time ago.
Any suggestions would be welcomed... Thanks