I have never tried this, but someone told me a cheap easy way to get flames out the exhaust.
Drill a hole in the pipe, weld a threaded coupling that will fit the threads on a spark plug, thread it in and hook it up to your coil where it gets spark. It will burn excess gas fumes that come out the exhaust.
Again I have never tried this, as I'm thinking about it now, if you had it hooked to say the #1 wire it would only flame when #1 is firing...... and if you have your carbs adjusted right, you shouldn't be running rich... So I don't know if this idea would work or not. Some guy I worked with way back told me this trick, guess you could put a switch in-between where you could turn the spark on and off....