Dip in timing

[J-440]

Got a FAST standalone with their duel sync distributor. I’m building a base timing table and have a nice even slope on the 3D graft. My problem is that I start off with 20* on start up and at idle. Then when I give it some gas the engine backfires but when I reach over 2500rpm she smoothes out. It’s like there’s a “dip” in that small area from 1500-2400rpm. I have the dizzy installed and timed correctly and my base fuel table is spot on.

Thanks