UPDATE: 04-06-2016
I have filled up around 4 times and have been careful not to fill the filler neck to the top by stop as I see gas bubbling up. When I look at the area above the dipstick I no longer see what looked like oil stains. Also, hot starting seems a little better but the weather need to get hotter to be sure.
I just had this problem and realized what was going on with my 2005 Grandvista 250. I chronically fill to the top. Last night I overfilled to the point that some gas spilled from the top. Next day the scooter started right up no problem. I let it warm up and shut it off. Restarted and went to the gym. Scooter sat in the hot sun for several hours. Could not get it started even with the throttle twisted 1/2 way. Then I noticed a big puddle of gas on the ground coming out of the bottom hole that is ~ 1/2 inch wide. Verified that all lines were hooked up correctly. Removed the carb and cranked the motor to dry things and hooked up the carb and got it started and put the seat back in. Got home ~ 3 miles. No gas leaked out.
Searched google which led me here and then I realized that last night I over filled and probably got gas into the charcoal filter. Also, I ride 5 days a week and have 19K miles so I always fill to the top then carefully tilt the scooter which causes the gas to go down into the filler neck to get the maximum distance out of each tank. I'm going to stop doing this.
The manual warns against overfilling but I though I was getting away with it. I also noticed on my scoot and another that had 26K miles that there is a stain under the carb and lower right side of the motor that looks like oil but is actually gas. My and probably the other scooter owners chronic overfilling may be the cause of this issue. I will clean that area, and stop overfilling and see if it goes away.
Could this also contribute to the hot starting problem where you need crank the motor with the throttle at ~ 1/2 way open?