I have a 390fe that's completely stock minus an HEI distributor and a 4 barrel intake and holley carb. With my basic 600cfm carburetor, a problem arose where the acceleration from idling around 700 rpm to about 1500 rpm was real fluttery and loud, and really weak. After 1500 rpm, it would instantly smooth out and cruise just fine. The problem gets worse as the car warms up to normal temperature. It also happens with hard acceleration at any rpm. I recently upgraded to a 670 street avenger, I got it used but completely rebuilt it and replaced the main body. The problem's still there. I've messed around with float level, power valve, and jetting. Where should I go from there? I'll look into my accelerator nozzle and pump cams next, and maybe plug wires because it sounds like it could be missing sometimes.