Ok I have a 72 460 that had a holley 750 on it, was told would get more power with a double pumper with mechanical secondaries. So I got a 850 double pumper, rebuilt it, 80 for primary and secondary jets, installed and it hesitates like crazy under load, idles fine. Timing is set at 15 degrees. Any ideas why I would be stumbling under load? Should I jet down?