I have a craftsman riding lawnmower, as im mowing the lawn little by little it looses power.?

it then shuts off. i have to remove the air filter to add little gasoline to the carb. and then goes back on. i do notice that when it shuts off, the fuel filter dries up of fuel. when i start it back again it slowly fills up again but does the same thing again. could it be that tank that is dirty? or the fuel line that is clogged?