My Wagon Master furnace
had the temperature control sensor mounted below the furnace
at the floor causing the burner to run continuously.
This must be a common problem with gravity furnaces specially when someone with no experience installs a used one with no instructions and not a whole lot of brains.
The external (outside the firebox) thermocouple must be positioned part way up the furnace
burner box as it regulates the temperature of the burner box so the temperature of the exhaust does not exceed safe levels. This will cause the furnace to cycle hence limiting the overall BTU output of the furnace.
Took an afternoon of tinkering but it now has the Fire box thermocouple mounted securely in the right location and it works great.