What is an air shutter?

Correct combustion needs the right air-to-gas ratio, and the air shutter is the adjustment that sets it. Too little air gives a lazy yellow, sooty flame that produces carbon monoxide; too much gives a noisy flame that lifts off the burner. A healthy natural gas flame is blue with defined inner cones. Persistent yellow tipping after cleaning means the shutter, the orifice size or the gas type is wrong.