Hi Glenn.Thanks for your reply here are 3 pics of the forge (as you can see a home made one) this shows the hood which is about 20inches above fired coke area.Hood measures 22 inches width by 17 inches depth .The forced air enters from the side via a 2inch diameter steel pipe. The fire area is sourounded by soft fire bricks as shown. I use these to contain the heat when forging knife blanks.
Hope this will sheds some more light on the problem.
I thinkk the volume of air delivered may be a contributing factor-may also need to throttle this down a small amount. Any more ideas guys.
Many thanks in advance H&T