Combine juice, sugar, soy sauce, vinegar, ginger, garlic powder and pepper in crockpot. Add roast, turning so it is fully coated.
Cover and cook on low for 7-8 hours.
Combine cornstarch and water in a small bowl. Add to saucepan and add juice from crockpot. Bring to boil, then stir and cook for 2 minutes, or until sauce is thickened.