April 11, 200917 yr Ken: Instability after that long a run is probably due to over heating the burner. Perhaps the interior of the furnace is getting hot enough radiant heat is enough to ignite the fuel air mix. You can try a couple quick things to see. First is cool the burner tube, a wet towel will do the trick but be VERY VERY careful. Nothing burns like steam so use a pair of tongs or something to wrap the wet rag (cotton) around the burner tube. If it's radiant heat then you'll need to make it turn a corner so the jet isn't looking straight into the forge. You'll need to make the tube diameter increase one step going around the bend and even then tuning might take a bit. Frosty
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