I would check the fuel cap. I had a Kawasaki 110 that had the on/off switch on the fuel cap rust out on the inside, luckly it was broken in the on position. That is one of the reasons that I use clear fuel line on my trikes.
Try rideing it until it shuts off then slowly unscrew the fuel cap and listen for a hissing. It does sound like you are running out of gas so also check the fuel filter. Gas can go bad fairly fast so it could be dirty.
-Andrew
1984 Yamaha YT 60 Tri Zinger(current project)
1983 Yamaha YTM 200 (rear suspension kit)..... long term project
2004 BMW R1150R
1972 Honda CT 70
2007 Lifan 110