Nice looking smoker... I too built a 120 gal smoker. ... http://www.smokingmeatforums.com/t/255846/120-gal-reverse-flow-not-your-normal-build-done
when I ran my numbers through the calculator... it came up with a 4" exhaust at 32" above the top ... being you went 6 1/4" .. try to close it down a little and see what happens ... can't hurt on a dry run ...