For the sauce, in a medium saucepan combine the ketchup, water, onion, vinegar, molasses, Worcestershire sauce, chili powder, and garlic. Bring to boiling; reduce heat.
Simmer, uncovered, for 10 to 15 minutes or until desired consistency, stirring often. Trim fat from ribs.
For a charcoal grill, arrange medium-hot coals around a drip pan. Test for medium heat above pan.
Place ribs, bone sides down, on grill rack over pan (or place ribs in a rib rack; place on grill rack)
Cover and grill for 1½ to 2 hours or until tender, brushing occasionally with sauce during last 10 minutes of grilling.
For a gas grill, preheat grill. Reduce heat to medium. Adjust for indirect cooking.
Grill as above, except place ribs in a roasting pan, and place pan on grill rack over burner that is off. Serve the remaining sauce with ribs.