I had a problem like that with one of those Webber carbs. I fixed it by overhauling the carb - new gaskets, needle valve, diaphram. I think the problem was caused by the needle valve. What was happening, was that the valve was still allowing fuel into the float chamber when it should not have - so the fuel level was too high. This was not a problem when the engine was running, but switching it off, leaving it for a few minutes and the extra fuel in the float chamber was gradually leaking into the inlet manifold causing the engine to flood. Warm the engine up, then take the air inlet pipe off the carb, open the throttle and see if fuel is dripping into the inlet manifold (ignore the fuel that squirts out as you open the throttle).