Trim all excess fat from roast. Place potatoes and onion in bottom of crockpot. Make a smooth paste of flour, mustard, chili sauce, Worcestershire sauce, vinegar and sugar. Spreadover top of roast (cut roast in half, if necessary, to fit easily). Place roast in crockpot on top of potatoes and onions. Cover and cook on Low setting for 10 to 12 hours (on High setting for 5 to 6 hours). (about 3 quarts).