Why won't my Buick start when tank is 1/3 or lower? on 1992 Buick Century

The car starts fine when the tank is full, but at 1/3 full or less, cranking the engine has no effect. If I crank long enough, I smell fuel. If I wait 15-20 minutes, the car will start, but then runs very rough. It eventually smooths out when it warms up. Any ideas out there?

Asked by for the 1992 Buick Century