250 with 2 Barrel Holley Won't Run

[hawker977]

I built a 1970 C-10 for my son a few years ago. I rebuilt the original 250 in line 6 and kept the Rochester Monojet on it. My son has had a lot of issues with the monojet and decided to upgrade to the Holley 2 barrel using a adapter. As soon as the 2 barrel was installed the trouble started. The mechanic said it would idle great, but coming off idle it would stumble and flashback through the carb. It also had little to no power. He worked on it a while and finally sent it to a dyno shop for tuning. They're having similar problems and are now looking to "rotate" the carb to get better flow into the manifold. I've never tuned a Holley so I got a couple of the SA type books on rebuilding and tuning Holleys, and I've watched a ton of videos including from Holley. Honestly, I don't think this is a tremendously complicated problem (I might be naive here). I am on the east coast and the truck in on the west coast so I can't get my hands on it. Has anyone out there had a similar problem where a 2 barrel on a 6 just didn't seem to work? I'm not sure of everything that has been tried. Based on my research, I would be sure the accelerator pump linkage is adjusted properly, ensure the squirter check is present, up the squirter nozzles about three sizes, test it and adjust from there. I would also swap out the power valve based on engine vacuum at idle and under load. I just don't understand why this is so difficult. Thoughts/comments/help???