drill a hole in the pipe's tip and tap it. mount a spark plug plug in the hole. buy one of the flame thrower kits they have in hot rod mags. it will rig it up so that when you hit a button the spark with cut out of the engine (loading the exhaust with fuel), you rev it up, and they when you let off the button it sparks the plug in the pipe and voila, FLAMES!
Well people do this to there cars all the time. weld in a bung for a spark plug and add an additional coil with a switch to ignite the unburnt fuel and there ya go. again like stated above if you have that much unburnt fuel then something isnt tuned right.