A toilet tank flush valve structure includes a valve base. A float assembly that is adjustable in buoyancy is movable between a sealing position and a flushing position. An overflow tube is attached to the float assembly and extends from a lower end in communication with a water outflow passage of the valve base to an upper end in communication with the interior space of the toilet tank. The upper end of the overflow tube is adjustable relative to the float assembly independently of the buoyancy of the float assembly. A guide member guides movement of the float assembly. A stop member on the guide member limits upward travel of the float assembly from its sealing position. The stop member is adjustable in position lengthwise of the guide member or by extension of the guide member independently of the buoyancy of the float assembly.