sounds like a clog in the fuel line or a problem with the float.
Had a similar issue when I put on a fuel injection fuel line I had for my Husaberg 450 on my WR450F (figured it would be a bit more heavy duty). Felt like my WR450F was running out of gas and running lean at higher speeds but if I waited a couple mins would start just fine and run at modest throttle openings. Thought maybe I had a partially clogged pilot jet or main jet Pulled off the fuel line at carb, turned on gas, plenty of gas flowed out. Hooked fuel it back up, still had the problem. Pulled off the fuel bowl drain and opened tank spigot...hardly got anything out. Figured some type of blockage in the float. What I ultimately figured out is that the inner rubber lining on the fuel injection hose was folding over and partially blocking the fuel flow when I hooked it up to the carb. Put on the cheap fuel line and it worked just fine.
Lesson here is to verify gas is getting to the carb first and then filling the float bowl. Could be a partial blockage somewhere along the way.
As Krannie indicated pull fuel line off and turn on gas to make sure it flows. I also suggest pulling float bowl drain and turn on gas to make sure it gas is making it to float bowl and filling it fast enough.
If that doesn't show the problem then I would continue items 3 and 4 that Krannie listed above.