watching and listening to progress,
did you remove and or completely block the functions of the old reed valve zone?
In the 45 years i've been tuning, I think what you are seeing now, with the lack of accell rise, and the backfire thing, is that things are going very rich, when a bike runs lean, it will rev sky high, (bwawwwwww), and when it is being over fueled, they bog..((braaaawwBoggggg), and fall flat..and won't high rev.
The backfire, is un burnt fuel, which ignites, when it finally mixes with air, in the exhaust, as secondary sign..
I also see you are running big pod filters, so, that may be adding some oddity.. maybe wrap them in a single layer of porous foam, to add some resistance for airflow.. it may have an effect on "back off "throttle fuel atomisation..
also glad to see you finally adjusted valves.. that should have been done from the beginning, as everything you have done relies on those.
just tossing some tidbits.. but I don't think you are really running lean, remember, your valves, and timing.. are not the same, as the EFI bike your injectors came from. If you compare the C14, which has larger displacement, a lot of fuel flow when desired, and much smaller exhaust pipes and restrictions, has a huge airbox where the filter lives.. it's function is to contain, and prevent reversion/pulse effects, of air supplied to the intake.