Cook onion, celery, and potatoes in enough water to cover (may include clam juice), simmering 20 minutes. Melt butter in sauce pan. Add flour and mix until smooth. Add milk. Cook over medium high heat until sauce is thick and bubbly, stirring constantly. Add pepper, sugar, and salt to taste. Mix white sauce into vegetables, and add clams. Makes a great potato soup if you don't like clams.