Is the fuel cap vent hole plugged? Sounds like a fuel supply problem. I'd disconnect the fuel line at the carb, turn on the fuel, and see if it runs freely.
Second thought, and I'm not being sarcastic, but are you sure you have ENOUGH fuel in the tank? It might be lower than the first stage fuel outlet. Turn the fuel knob all the way open to get the rest of the fuel out by accessing the "reserve."