The only time I used the damper was when I wanted to smooth out a temperature spike - and I didn't leave it closed for long.
Me, I'd look at your pit for air leakes that might be keeping your temps down. Rust holes, gaps where metal meets metal, etc., then plug the leaks. Use lump charcoal like everyone else has mentioned, with some wood on top of that. If the pit is in a windy spot, either block the wind or move the pit. You'd be surprised how much that helps, especially when you've got air leaks.
NB El Dorado Offset, Primo Oval Junior, Traeger Model 075 Pellet Pooper, Big Chief, Kingfisher Kooker 14" gas/wood combo, PBC Brethren Edition
Home Brewer, Murderer of Squirrels, Armadillo Inspector
Flaming Pig Head Mod
Black (Stealth) Thermapen Driver - you saw NOTHING
Certified KCBS BBQ Judge