The pilot was providing for hire sightseeing tours of a scenic area of Sedona, Arizona, in a vintage biplane. The front cockpit had been modified to accommodate two passengers. No record of an approved seat modification in accordance with factory specifications or a FAA field approval was located. The pilot stated that he departed with 10 gallons of fuel on board for a 30 minute tour. He could not say how the fuel was distributed in the tanks. He also said he did not use a dip stick or other measuring device to quantify the fuel level, but estimated the total fuel based on consumption estimates from prior flights. The airplane has two 25 gallon fuel tanks in the top wings with float sight gauges underwing. The aircraft maintenance manual states that 10 gallons of fuel is the minimum required for takeoff in the tank selected. Regarding the sight gauges, the manual states: 'An accurate reading cannot be obtained from the gasoline gauges on the ground'. Both gauges were contaminated with a tar like substance adhering to the wire indicators, which resembled an unidentified coating found inside the fuel tanks. Examination of the fuel system disclosed that only residual fuel remained in the fuel tanks and lines to the engine. Fuel system continuity was established.