that's true. a well used bike could have crap clogging the in-tank fuel filter.
pull the fuel line off the carb and turn the fuel petcock on. there should be a healthy flow there.
also make sure your fuel tank is breathing. if the problem goes away by taking the cap off then you'll need to drill a hole into it to let the tank breath (incoming air takes the place of the gas that went to the carb. otherwise a vacuum happens which prevents fuel flow.)