Take the fuel line off at the carb and make sure you have good fuel flow. Drain the tank out threw this line with the fuel tank cap on. This ensures that the line is clear, the vent in the tank is working and you will be able to see if there is any debris in the tank after it empties.
If that all checks good I would take the carb off and see if the floats are sticking and that the carb and jets are clean.
Also check that you do not have a intake leak between the carb and the cylinder head.