the seat will have an o ring or gasket where it goes into the carb body. the inlet needle will have some sort of coating at the tip.
more often than not, you will find some debris where the needle sits in the seat affecting the ability for it to properly close off the fuel flow. start by removing the needle and checking.
next, remove the seat. should be held in place with a small tab and a phillips screw. careful, those screws are easily stripped. remove the seat and check for obstruction, check the o ring, and check the supply circuit(where the fuel comes in).
on the seat should be a number casting. break out the magnifying glass. this will be your reference number to search for an aftermarket one. oems are amazingly expensive. needle and seat will come as a set.