Hi Andrew
I have the same problem on one of the carbs I have in the garage. One of the floats had been repaired and it doesnt run at all well. It appears that if you start using glue or something to fix the metal bit to the top of a float, it affects the weight and then it overfills, floods etc.
Luckily I had the carbs off my old 2Litre engine and although they have the smaller bore, they run fine so I don't need the spare 240Z carbs at the moment.
I do still have one good carb/float so I could let you have that. It would probably be best to send you the whole carb.
If you are interested, PM me.
Paul