Is the tank plug in line with the carb? If the fuel tank plug is lower than the carb? readjust if needed so that it is even or slightly above the carb throat. I have also seen the carb throttle lever come loose. It will still move as if it's working as it should, but the carb guts will move only slightly when the lever becomes loose. So it will seem that you should be pumping fuel when in fact, just the lever is moving and nothing else.