I bought a Weber Smokey Mountain 18.5" today and wanted to do a test run with no meat, just charcoal to see how my temperature ran. I filled up the charcoal ring in the smoker and filled up my weber chimney up to the top of the handle. Once all of my coals on the chimney were lit, I dumped them on the rest of the coals and spread evenly. I left all of the vents open to see where I started and slowly started shutting off the bottom vents. The lowest I could my smoker to was 255 and that was with two vents closed on bottom, the other bottom vent 3/4 closed and the top 1/2 closed. What did I do wrong? Does not having meat or water in the smoker effect this? Help please!!