Jesse James did it on..., I think it was the Mustang Lawnmower! They injected a small amount of either propane or butane (I can't remember which) just before the tip of the exhaust and put an ignitor (spark plug or small open flame) after that. Then when they wanted to shot flames, they turned on which ever fuel it was and lite the ignitor.
IIRC, they had a valve set up that would regulate the amount of fuel for the flame depending on the engines RPM. In other words, you could not shot flames with out mashing the gas pedal. When the engine would idle, there was this solid flame that would come from the pipe. Hit the gas and BOOM! Instant roasted marshmellows.
I know it is a lame description of what they did, but it is close enough. I guess you will have to watch the rerun to get more details.
