Combine garlic, broth, marmalade, vinegar, and catsup.
Put a large plastic bag in a rimmed baking pan.
Place rib slabs in bag, pour in marinade, and twist-tie bag closed.
Refrigerate until next day, turning bag over occasionally.
Arrange enough low-glowing coals to form a solid rectangular bed about 6 inches across and extending 3 to 4 inches beyond ends of ribs
Coals should be about 2 inches behind an imaginary wall extending from spit to firebed.
Place metal drip pan directly beneath and in front of spit.
Lift ribs from marinade and drain briefly
Passing the spit through meaty sections, thread ribs back and forth in an s-shaped pattern.
Position spit on barbecue and start motor.
Cook, basting occasionally but sparingly with reserved marinade, for 1 to 1 1/2 hours or until meat near bone is no longer pink when slashed.
Every 1/2 hour add 5 or 6 briquets to fire, spacing them evenly, to maintain a constant temperature.