Yes, a worn needle and seat will cause gas to get into the lower end, especially if you leave the petcock on the "on" position when you are done riding and let it sit. The carb will just keep filling slowly, and it will leak past the piston rings and into the bottom end. To check the gas cap vent, you can try blowing in the holes on the bottom side of the cap, and see if anything comes out the top.
THE 80's RAGE! Currently owns:
81, 85 Yamaha Tri Moto 125
83 Yamaha Tri Moto 200 x2
82,83 Yamaha Tri Moto 175 project
85 Yamaha Tri Z 250
84 Kawasaki Tecate 250
84 Honda ATC 70
85 Honda ATC 350X
84 Honda ATC 250R
(2) 84 Honda ATC 200x
82, 83 Honda ATC 185s
81 ATC 200
Lots of other parts and bikes