How much bigger than stock are your tires and what rear end gearing do you have. I know when I went from 265-76-16's to 285-75-16's my truck fell on its face off the line. I went from 13.6 to 14.2's from that small jump. If I am not mistaken you have made a much bigger jump in size than I did. Even when I added bigger injectors and a Smarty to my O1 I could not over come the larger tires.
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2001 2500 4x4 QC/LWB - Mach 4's, Drag Comp, Smarty and all the stuff needed to run the mods.
13.69 @ 98 mph before the Mach 4's and Smarty