Steve,
I could not reply to the original post, but I think your problem is with the Carb Diaphragm I had this problem with the rotovator I bought off e-bay after changing the diaphragm it ran ok because the engine was 1976 vintage it was also knackered, but since then I have had a replacement engine fitted a Briggs&Stratton 5hp Quamtron and it runs like a beauty, cheers