How to get heat down in a Great Smoky Mountain Propane Smoker

Open the top vents, this will let heat escape and gives good airflow through the smoker. You could also try installing a needle valve, that will help adjust the gas flow. The valve on the tank is either open or closed it really doesn't give you any adjustment. This will show you all about the needle valve:
http://www.smokingmeatforums.com/a/needle-valves-for-gassers