like others have said, it takes stomping the gas to get a flame thrower workin by just puttin a spark plug in the gas.
if you would like to be able to throw a little (1-8 foot) flame, anytime you want to, run a propane set up. set the spark plug and the energy source up the same exact way, but instead of using the throttle to start it, you use a switch attached to the dash. i use 16 ga. wire all the way around on every car and truck ive done. to make things easier set the propane up with a sylinoid that is also activated by the same switch, and you will be good to go anytime you feel like. just flip the switch and watch the heads turn.
best way, is if you have a truck, run 6 inch stacks, or whichever you prefer, you will get a louder rumble, and MUCH larger flames.