This is a Burner running at low pressure and burning a bit yellow. It seems that in every batch a few run a bit rich at idle. I've never had one that didn't burn totally blue at above 5 PSI though. If you do you can always press out the plug and replace it. I suspicion that the orifice drill cuts a bit wide now and then. This is a square law world and the change in area from 0.026 to 0.028 is quite a lot. If you consistently get rich ones you could use a #70 drill. Also make sure that the air mixture holes are completely de-burred.