make sure you turn gas cap till it fully locks,silly ,but it happens.or try different gas cap,local auto parts store,motorcycle shop or farm supply take your old one to compare.could also try adding or using thicker rubber seal.mine had oppisite prob too much seal not allowing tank to vent,therfor no fuel flow.hope this helps.
If it is leaking out of the gas cap vent, this is common. Alot of folks only fill the tank to 3/4 full to prevent alot of the splash up into the cap.
Another problem that some of the caps have is there is no direct way for any gas trapped inside the cap to drain out. Drilling a 1/4" hole in the center of the cap bottom will fix this.
The last common problem is the gas cap gasket has a poor seal to the tank filler neck and gas leaks out this way. Some folks use 2 gaskets on one cap, some folks cut a new gasket out of leather and add it to the factory gasket or use it by itself, depending on the thickness of the leather.