Bank about 20 medium-glowing coals on each side of the fire grill and place a metal drip pan in center.
Place grill 4 to 6 inches above drip pan; grease grill lightly.
Brush lamb with a baste or, if marinated, lift meat from marinade and drain briefly (reserve marinade).
Place meat on grill, directly over drip pan.
Cover barbecue and adjust dampers according to manufacturer's directions.
Cook, basting occasionally with reserved marinade, for about 2 hours or until meat thermometer inserted into thickest portion registers 145° for rare, 155° for medium.
Add 5 or 6 briquets on each side of fire every 1/2 hour to maintain a constant temperature.