Ultralights under FAR 103 are regulated to a capacity of 5 gallons. I'm wondering how exactly fuel capacity is defined, does it include fuel in the lines, and in the carburetor bowl or elsewhere or just the tank feeding the engine?
FAA Advisory Circular AC 103-7, Paragraph 19 has this to say:
MAXIMUM FUEL CAPACITY OF A POWERED ULTRALIGHT VEHICLE.
The maximum fuel capacity for a powered ultralight vehicle is 5 U.S. gallons. Any powered ultralight with fuel tank(s) exceeding this capacity is ineligible for operation as an ultralight vehicle.
a. Determination of Fuel Capacity.
The total volume, including all available space for usable and unusable fuel in the fuel tank or tanks on the vehicle is the mtal fuel capacity. The fuel in the lines, pump, strainer, and carburetor is not considered in a calculation of total volume.