Biohazard

Engine wouldn't stay running, and an unburnt A/F mixture wound up shooting back through the intake when the engine came to a stop (more accurately, reversed slightly due to compression in the other cylinders)?

How's your spark and timing? Any reason either would be giving trouble now as opposed to the last time it ran? My guess is this would have been a backfire had spark been OK.

By the time that plume came out of the carb, the engine was no longer running; all the spinning mechanical sounds were from the starter.

-Kurt