Then consider that the amount of pressure in the fount effects the amount of fuel delivered, yes. But the design of the burner accounts for this with induction, the more pressure the higher the fuel vapor velocity from a given orifice will induce more air to be taken in the air/mix tube. Within the operating limits, the burner is designed to match the fuel air mixture with the amount of induction from the velocity, the mixture stays the same percentage, even as the volume of both air and fuel change. The size of the nozzle (jet, tip) sets the ratio to suit the design and fuel type used, and of course a change in orifice size can be used to adjust the ratio of fuel to air used. Different brands and designs can vary but they all use the same principle. Hope that helps.
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