I think i figured out how the fuel ends up in the engine. When the diaphram breaks down and leaks the fuel finds the hole and drains the tank into the engine through the vacuum port on the manifold. Or if bad enough and the tank is full up and out of the carb with fuel flowing out the overflow tube and into the intake manifold at the same time due to the high pressure in the tank.

If the vent hole is plugged or swollen up due to ethanol poisoning the pressure in the tank and be real high on a hot day so it actually blows the fuel into the engine....