[concobra]

I did not read through all the responses but I will share some thoughts

1. Over rich and retarded timing can contribute. But rare
2. Exhaust leak is more than likely the problem. This is always accompanied with a back fire. You did not write of a back fire though.
3. If you are running a holley carburetor this can occur. If the back float level is set high and you are braking hard. (That is what you are seeing when you mentioned seeing racing cars do this) The hard braking causes the fuel to slosh up against the metering block in the secondary float bowl. The fuel overwelhms the secondary circuit and spills out the booster venturies causing a super rich/flooding condition. This combined with an exhaust leak can cause your flaming issue. The fix is to simply lower the seconary floats level. I set mine about 1/4" below the sight hole. Double check your fuel curve on your plugs as this can lean out the secondary circuit.

I hope this helps