Gas tank venting: very important. Normal is to have a valve that lets air in as gas is used. A problem can arise with this if you store the car with partial fill, so there is air/vapor in the tank and the car cools down, like in winter. Air goes in. Then spring comes, and the car warms up. Air and vapor try to expand, but can't. Pressure builds and can push gas into the carb, past the float valve, down the throat, past an open intake valve and past the rings and into the crankcase. Can fill the crankcase. Then you start the engine, and the crankcase is full of liquid, some oil and lots of gas. This builds enormous pressure in the crank case, and that very thin mixture goes right back up past the rings and/or right up the crank vent which is often routed right into a carb or intake manifold. And then the mixture gets burned into a smoke cloud the likes of which you probably have never seen. You could ask me how I know this, but I just told you.