Old propane floor furnace ignition problem

We have an old (40+yrs) 'Fairway' floor furnace. It is cleaned regularly and pilot and thermocouple are functioning. When the thermostat calls for heat, the ports directly at the pilot light ignite and then after a 4 or 5 second pause, the rest of the burner lights with a "whump" and the air inlet where the gas nozzle injects, also lights with a very loud growl. This is in the casting where the butterfly controls the air inlet. Our service man is puzzled but says as long as the back-fire (his term) is well in the casting, it's not dangerous, just noisy. Any ideas how i can stop the flame at the gas inlet?
"confused in the Adirondacks"
bilrich

 

I'm sorry, but I disagree with him. Sooner or later the flame will start to leave the combustion box. Has he removed the burners and THOROUGHLY cleaned them [cleaned out each slot? They are probably old cast iron ribbon burners] ? Ditto for the gas jets, heat exchanger, crossovers, and flue? I would hope he has checked the gas pressure?

 

daddyjohn, thanks for the info. I was with the service guy and we removed the cast iron burner and cleaned it. Some of the flame ports were clogged and we drilled then out. The burner is one piece, about 30" long with the combustion chamber branching off from the center. When the burner is placed on end, it resembles the lower case letter 'h'. The entire burner unit has been blown clear and the accumulated rust and dirt was vacumed out. When the burner is lit, all jets burn properly with a good blue flame, unfortunately the air/gas chamber also burns. Any other ideas?
bilrich

 

Was the gas orifice futzed with? You only mentioned cleaning the burner. What about the heat exchanger, flue, etc.? It's little bit of a head scratcher but the flame should only be at the burner. Air should be mixing with the gas then moving on to the burner. What happens if you try adjusting the air shutter?