I load my ECB (electric Gourmet converted to charcoal) with 1 full chimney worth of briquettes and using the Minion Method it'll hold 250F close to 10 hours. Lid is sealed with rope gasket and the 3/4" vent holes I made on the lid are always fully open.
My charcoal pan is well sealed with one single adjustable vent.
Also make sure you're not getting any air leaks through the access door. A leak there will keep temps high no matter what you do.
....UDS....Mini-WSM....Midi-WSM....Mod-ECB....22.5-OTG....34"wood fired pizza oven....