Ok, My brother just switched to a '66 carter 4-barrel on his '66 Catalina 389 and it has a flat spot under light acceleration. Someone had said it might be the accelerator pump but I don't think so because you can hold it at the point where it stutters and it will keep stuttering. I tried putting new jets (edelbrock #1426 .095 orifice) in it and forgot to put the rods in and it ran good but when I put the rods (edelbrock #1455 7042) in it was back to stuttering. The step-up springs are edelbrock silver 8-in. Hg.