I'm not a plastic carburetor guy Steve, so we need one to help us out here. However you may be able to sort it out experimentally if necessary. Can you remove the front cover of the float chamber compartment to reveal the float, with the carburetor still on the mower? Just lift up the float gently with your finger, and turn on the fuel. Remember it's a tiny float, so only apply a tiny amount of force to the float, just enough to overcome its weight. If fuel flows through the needle and seat, you'll have proved that it isn't working. Then experiment to find out why.
