Bring a large pot of salted water to a boil. Add potatoes; cook until tender but still firm (about 15 minutes). Drain, cool and slice potatoes. Place bacon in a large, deep pan. Cook over medium-high heat until evenly brown. Drain and set aside. Reserve bacon fat. Add flour to bacon fat, mix until smooth, then add sugar, water and vinegar. Cook over medium heat until dressing is thick. Add bacon, potatoes and green onions to skillet and stir until coated. Cook until potatoes are tender and serve warm. Season to taste with salt and pepper.