My vote is ethanol, try 6 weeks is all your gas will be good if you are using a carb, all the while its eating the rubber like Roger said. The reason you efi car can go longer is the engine is adjusting the mixture sensed by the O2 sensor compensating for our crappy fuel we buy. If your buying from a station with above ground tanks 6 weeks is a over estimate as the heat breaks it down even quicker, I talked with a expert on this, he is a car collector and owned a oil company, makes a product to use on ethanol fuel in older cars and still said "Buy ethanol free" here you go, after the carb rebuild Ethanol-free gas stations in the U.S. and Canada