Originally Posted by Pappers_
I don't know what the venturi bell is (and google didn't help) - is that the piece where you control the air/oxygen flow? [...]
The piece that controls the air flow is a damper. The damper covers the mouth of the bell-shaped induction manifold. The pressurized gas passing through the flare fitting in the center of the damper pulls air through the damper openings, a phenomenon caused by the venturi effect - which occurs within the "bell".
Not all of the required make-up air for full combustion at the nozzle tips is provided via the induction system - additional air is required at the nozzle tips. If this wasn't the case you'd always have a roaring fire within the manifold (not good).
When the gas flow declines too far the air/fuel ratio within the manifold will approach the level required for combustion, and you'll get the "back fire" effect.
Thus I expect this was yet another case of the contents of the propane tank getting so cold the evaporative pressure dropped too low to maintain the excessively rich air/fuel ratio within the manifold, and it lit up...