Switched to a 100 jet and the backfiring is basically gone. Why would a 120 jet be causing the flames when most people recommend that as a good jet size?
I can give an opinion on that.
The people recommending a 120 jet on a 196 may be running 5K plus RPM and may be using a cam. A 120 jet should work better at 5K RPM full throttle than it would at 3400 RPM and full throttle. The 120 jet may also be their starting point, and they dial it in from there.
The 120 jet may be useful if you had the stock jackshaft on it. But you would surely run it into valve float and too much RPM for the stock rod. And the 18 pound springs some guys recommend would make matters worse for the stock rod.