Evinrude carburetor overflowing.

This is my first post and I'm in Sweden so I hope I'm using the correct terminology. I'm helping a mate with a 40 hp Evinrude from 1963. It starts perfectly, it runs perfectly, it accelerates perfectly, but when I throttle down from highish revs it overflows (floods) so that gas literally pours out of the carb. I've cleaned it meticulously, installed a new float valve, adjusted the float (which seems to be in good condition) as recommended. My theory is that when I throttle down an excessive vacuum is generated in the inlet manifold which the float valve can't cope with and lets gas be sucked up uncontrolled through the various jets. Any suggestions?