The possibility that seems to stand out, since you've just replaced the seat, is that the float level is now higher than it was. I gather you now have a seat from one model carburetor and a needle from another. Let's see if there is a way to adjust the float height. It is usually done with a metal tab at the pivot, which can be bent. Don't adjust it though until you get a picture of the tab, and a measurement of the current float height.
