The burner is designed to burn a yellow/orange rolling flame to create the perfect cooking environment for pizza. If the burner created a blue flame, it would simply burn too hot. Because of this, there is an incomplete combustion and a soot build up is expected. If the soot build-up is not favourable, you can use a flue extension.
With a more efficient setup, the flame is much shorter with bluer and less of the orange/yellow tips and this produces a locally hotter, but limited heat source that leads to a very uneven cooking. By reducing the air in, we get a longer flame that lick across the oven dome with the brighter yellow tips. This not only heats the oven up more evenly before cooking, but also during cooking the pizza will cook evenly and only require moving a few times. It also provides more light within the cooking chamber so you can easily see your pizza being cooked. It’s very much a balance between the best cooking experience and having soot deposited on the outside – it should be noted that wood is far worse in this case than gas and LPG worse than natural gas.