Have you checked the small choke et in the bottom of the float chamber? Take the loat chamber cover off and invert it. There is a cylindrical chamber at teh back where the choke tube attached to teh main carb body fits. At the bottom of that chamber is a tint brass jet which can block up. Try applying an air line to the top of the cylindricasl chamber - that might dislodge any blockage. Otherwise, look at the botom of the cylinrical chamber and you'l see where the brass jet feeds from the main float chamber. Try poking it with a thin, stiff piece of wire. I use a pricker supplied for clearing jets in camping gas stoves.
Alternatively, just try fitting another Float chamber complete!.
Hope te helps,
Graham