Can you post a picture of where it screws in please? It sounds as if it is an emissions carburetor, and it is quite correct for it to be fully screwed in, because the idle mixture is not adjustable.
Up to now I've expected to find that your engine was made sometime in the 1980s, which is why I've been accepting the theory that it had adjustable idle mixture. Your engine was actually made on 14 March, 2002. I thought Rover 45s were out of production by then, which would make it a replacement engine, but Deejay is the reel mower guy, I'm sure he'll put me right as required.
