I can only speak from modern experience, a 12 month period of not running of a Briggs and Stratton engine with test hours only. On a horizontal band saw, left 1/2 full of E5 petrol. When I came to start it several attempts proved fruitless. So I drained the tank cleaned out the brown gunge in the carburettor, refilled with fresh petrol and second pull off it went. I think the fuel in the tank was separating into layers of its constituent parts. This is just my thoughts and experience of using modern petrol.