I've attached a neat graphic I ran across that shows the relative heat value of propane vs. natural gas. Many of the old stoves had an orifice sized for natural gas. When you try to run propane through it it lets way too much through. That's why you need to replace or resize, if an adjustable orifice, to use propane.
Here's the text that went with it.
"The additional hydrogen and carbon atoms in propane also mean that it produces more heat than natural gas from the same volume of gas burned at the same efficiency. Orifices for appliances burning propane are therefore smaller than orifices for natural gas burners (and also because propane is delivered under higher pressure, see above)."
