You may have the airstream too close to the coals? If moving it further back is not an option, try placing a metal grid or grating where it will break up the airstream. I take it you are burning charcoal? What kind? Smiths in the old days used to modify the heat output of their fire by several methods: changing the depth and shape of their firebox, making the actual bed of coals deeper or shallower, adding wood to the coals (preferred woods were hawthorn, oak, ash (NOT rowan or mountain ash wood, but the actual ash tree, Fraxinus, the one with the little rotor-looking seeds called ash keys.). Hawthorn burns very hot, oak and ash cooler but even and long-lasting. Make sure the wood is dry.
Really the only way to work out what works for your forge is to experiment. Have fun!
