As you said, the longer needle together with the new seat would have made the float lever geometry different, which might have caused the float to exert more closing pressure on the needle against the seat. However if you bought the new needle and seat as a set, and they matched your carburetor model, they should have worked. Perhaps your rather worn float pivot has developed a favourite place to sit, and binds a bit when it has to operate in a different position. Alternatively, the needle and seat you bought might have been intended for a different version of the LMS carburetor.
I'm glad it's fixed, vwq47. I'll close this thread - if anyone needs to post to it, just PM a moderator.
