Inliners International Forums General Discussion Engines 250ci stumble and backfire????????

Hello I-sixers
My chevy 250 with a rebuilt head, offy intake, DEMON 525cfm 4brl, and headers. Stumbles or backfires through carb under hard acceleration, and accelerates roughly. I have a stock cam and head (for now). I have tried a lot of things, like adjusting timing, accelerator pump and float leves, but it does not change much. Any info would be very helpful. thanks!