ATM50 sputters midrange

[Warpig]

I have a 2016 atm50 that I just installed a 80cc bbk last week. It's been running great until tonight. I took off from work and when I hit 10mph it started sputtering and backfiring until about 20 - 25mph then it took off. Did this every time I had to stop and start. Got it home and took it inside put it up on center stand and it did the same thing with no load. Also I noticed a slight flickering of my dash lights.
This is a brand new engine that i installed the bbk before i even ran it. New carb with a 90 jet also.
Something else i noticed. There was a slght flicker of my dash lights during the backfire and sputtering.
Any ideas before I start blindly taking it apart?
Thanks guys.
Just checked my spark plug. It is black. Wasn't like that 2 days ago.

[90GTVert]

Ignition issues could cause sputtering and plug fouling. My first check would be the grounds. Make sure they're all in place and secure. There should be grounds from engine to frame to negative battery terminal. Then check ignition system connectors at the CDI, coil, and stator.