You nailed it. The jet size varies based on the BTU potential of the fuel source.From my experience from working with LPG and converting back and forth between LPG and natural gas, each of them took a different sized orifice in the valve. If one hasn't used a kit or the right orifice, I would suggest checking to see what would happen with the wrong orifice. With such things as ranges, gas furnaces, and hot water heaters, it makes a big difference. As for feeding an internal combustion engine, I don't know if there is a difference between the carburetors of the two different gas types.