Why do flames come out the stack? - Trains Magazine

Decades ago, when left idling for hours in -35F worn Fairbanks Morse opposed piston locomotives would cool down so much that not all the cylinders would fire when fuel injected.

The exhuast beat would become ragged and the smoke a greenish-blue that would hide the streetlights in town at night, depending on the wind.

Had to be smelled to be believed!!

As the cylinders in an OP were vertical, the unburned fuel would eventually flow out into the horizontal exhaust manifolds at the base of the block, collect, and then drool into the vertical snubbers ( Mufflers ) at the end of the engine.

 Some of the fuel would always be 'gassing off' in the manifolds and snubbers, just waiting for ignition.

Open the throttle to warm up  the cab heaters or pump up the air, and BOOM!

Long tongues of flame out of both stacks and, possibly cracked snubbers.

Something to behold when competing with the Northern Lights!

For a while the Company fitted rounded screens to the tops of the stacks on OPs to cut down on forest fires from expelled carbon chips, and they, if fitted, would soar off into space, coming down, hopefully, not on one's head or the Loco Foreman's car.

In severe weather, locomotives were sometimes left in Run 4 with Gen Field off to keep them and the cab heaters warm.