A couple other ideas would be the vent in the fuel tank cap. Make sure its clear, blow it out with compressed air. If its clogged, the fuel wont flow through the line. Other likely cause would be the coil. Most of the time when a bad coil gets warm, it will not produce spark, or not enough to run properly. Im thinking it wouldnt be a fuel level issue at the float, as it would either run funny all the time, run out the overflow, or run out of gas at WOT.
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