Dual Fuel Burners

Frank_5

When installing a dual fuel burner using oil only, shouldn't the gas butterfly valve be plugged? If not couldn't ambient air be drawn into the open gas valve, through the orfices and change the character of the oil flame. If left open also could by products of oil combustion be forced down into the gas orfices and out the butterfly valve? The answer I'm getting from our oil techs is that these valves are usually left unplugged.