Andy has my vote also. ( Hate admitting that) If it runs outside then it is in airflow set up. If it does not, which I'll bet it doesn't, then its in the engine. Make sure you don't do your test run in bright sunlight. It may mask some flame you won't see. The most likely way you get flames out the back is too much fuel in the combustion chamber. Un-burnt and wet fuel into exhaust. RPM sensor, Fuel pump advance settings, would be my best guess.
Interesting and keep us up to date. Good Luck