I've just begun firing a small downdraft kiln I bought used. The flames go direct into the kiln fine when the air intake vents attached to them are closed. If opened more than 1/4 inch they start backfiring and I'm concerned about the flame being active outside the kiln. The initial solution is to not open the air intake valves more than 1/4", but then there is not enough heat combustion entering the kiln to fire it off in a reasonable length of time. I've tried lowering the gas while introducing more air, but this didn't help. Any suggestions?