My boat has 20 ounces(591.5mL) of fuel at full load. I run a 3 minute prerace "milling period" and a 3 to 5 lap race on a roughly .2 mile(.32Km) course. I use 50-55% nitro fuel and, with the milling period and 5 laps, I normally go through 18 ounces(532.3mL) of fuel. What you have to remember is that nitro is an oxidizer, not a fuel. It gives more power and allows me to run a bigger/higher pitched prop. If I were to break down what I'm actually carrying in the tank, I have

• 11 ounces(325.3mL) of nitromethane
• 6.8 ounces(201.1mL) of methyl Alcohol
• 3.2 ounces(94.64mL) of lubricant

Using those numbers, you would think medium would work but only about a third of the volume is fuel. You also have to remember that a smaller fuel line requires more pressure to push the same amount of fuel through as a larger one at lower pressure. Your YS 115 probably had little to no nitro in the fuel and was probably equipped with a fuel pump, something not used with marine engines