You need to get the model number of your lawnmower (the body, not the engine). Look up the parts diagram on a site like ereplacementparts.com It will tell you the parts number. FYI - there are often several numbers for the same parts, depending on who the manufacturer is (OEM or aftermarket)
If everything else mentioned here fails, there is another potential problem: WATER. Now that fuel has higher ethanol percentage, it attracts water. Sometimes there is so much water in the tank that the fuel pump doesn't actually pick up any fuel, but only water. Obviously a car cannot run on water. Simple solution - drain the gas tank.