Have the same problem. Took it into the shop and they did a coumputer thing with my 300i Downtown. You see my check engine light came on. They said that they did all the checks that the computer said to do. Only thing left would be to change the injector. Now I added some injector cleaner and I will do that for my next few fill-ups. I was also told by some friends that ride scoots from 50cc-500cc that they had the shop drill a hole into the gas cap and that problem went away.
Bought my 300i downtown last Nov.. Have over 6500 miles on it now. I'm 6"3 and 212 lbs. Other then that problem and 2 others, this has to be the best scooter on the road--for me!!
The other two thing I don't like, but can live with, are my seat and where my legs go. I get about 75+ mpg and it will do 93mph. If you play with it. I live in San Antonio, Texas.
I will let you know if what I'm doing with the cleaner works.
Ride Safe, Have Fun, but RIDE!!! Snowman