First, check to make sure its getting sufficient fuel flow. Disconnect the line from the carb, open the valve, should get a nice steady stream of fuel. If you don't, disconnect the line at the valve (if you have a fuel filter), and repeat the above test. If you still don't have good fuel flow, replace the shutoff valve.
If its not that, move into the carb. Take some carb cleaner and shoot into the fuel valve (the one that the float normally opens and closes), might be some debris in there blocking things up. Then try adjusting the slide needle clip position. Should be 4 or 5 slots for it to go into. Also, the main jet can work loose and fall into the float bowl, which will give you no top end (run VERY rich if at all above 3/4 throttle).