The step up springs are as follows;3"Hg (blue), 4"Hg (yellow), 5"Hg (orange), 7"Hg(pink), and 8"Hg(plain). If any engine produces less than 8"Hg barely off idle, it has serious problems. 1strodeo stated that his Bronco "drives great accelerates great" so it's a safe bet that his engine has a lot more than 8"Hg when running "at very light throttle". There are no stiffer springs than the plain or 8"Hg. The metering rods provide high load enrichment. An incorrect spring would have been a problem for driving and accelerating. 1strodeo says these were great so there's no reason to change them. The lean serge off idle never involves the metering rods or step up springs. The fact that he doesn't have an accurate vacuum gauge isn't a problem in diagnosing his off idle lean serge. A good idle mixture can be easily achieved by simply finding the smoothest idle. He has no complaints about his idle. There is no reason to get into ignition issues. If there is an ignition miss it would have occurred during acceleration. That didn't happen, as 1strodeo says his acceleration is great. I look forward to hearing what the tech people at Edelbrock have to say. I want to add that Edelbrock has an owners manual that is included with new Performer AFBs. It tells you how to use the calibration kit. If you don't have one, they are down loadable on their sight.