The "gas oriface" that is refered to in many discusison on this forum is at the end of gas hose (or hard plumbing) where it connects to the burner. For most burner/ gas hose combinations there is a female flare or NPT fitting on the end of the hose. The oriface fitting itself is a small brass part that looks something like a reducer or thread converter that screws into that. The oriface fitting is what threads into the burner right in the middle of the inlet side, in the same plane as the air adjuster disk if you will. If you unscrew the oriface fitting from the burner and look at the end you'll see a very small hole. That small hole is the actual oriface that everyone is talking sizes about. It's the primary control, along with regulator pressure, of how much gas enters the burner and probably more importantly what velocity the gas enters at. That is because these burners rely on gas velocity thru the throat of the burner casting to create a venturi effect that pulls the air in to mix with the gas. The air adjuster disk is then a fine control over the fuel/air ratio.
If you're familar with how a carburator on a gasoline engine work, this is the exact opposite. In a gasoline engine the piston downstroke sucks the air into the cylinder through the throat of the carburator creating a venturi effect that pulls the liquid gasoline into the air, creating the air fuel mixture. Here the carburator jet does the same thing as the propane oriface in the burner. In fact another name for the jet is, not surprizingly, an oriface. The more air that flows through the carburator throught the more gasoline that gets pulled in. In the propane burner the pressurized gas flowing out the oriface pulls the air in through the air disk creating an air/fuel mixture in the same way.
I hope that explains it a little more clearly. Once you gain and understanding of how these things work they're really surprizingly simple devices.
