Hi RobF, and a warm welcome to the forum. As a '80 Series Victa TAC owner myself, I know that they are an excellent machine, mate!
Never having this prob of yours, I can't help with a diagnosis, but I can tell you that it is a Model G4 carburettor assembly (Part No: CH03499A).
We are extremely lucky here at OutdoorKing to have many Victa aficionados and our Victa carby guru is Joe Carrol.

Someone will put you on the right track, Rob.
Once again,
