The washer isn't shown in the assembly diagrams as well as not being in the carburetor shown on that other thread, sparker. It has to be a seal, and its purpose may be to stop air from reaching the carburetor venturi without having passed through the emulsifier. If so it would go on the top of the rib around the emulsifier, near its top. However if it goes there it will have been compressed when it was previusly installed, and you can probably find crush marks on it, which should match both the emulsifier rib and the step in the carburetor body that it clamps against.
