[PeteF]

I just completed this process and finally got the secondaries to open properly. I have a 351W, 380HP, with a Holley Model 3310 750 cfm. The carb had the stiffest spring (black) installed from the factory. After trying the medium (plain) spring, and then the light (short yellow) spring with no change, I finally ended up with the lightest spring in the kit (white). Even with the lightest spring, the secondaries did not open on first try. I suspected the tie wrap may be too tight on the shaft (inhibiting actuation), so I removed it and used a paper clip, as recommended on the Holley website (installed on the right side vacuum actuator rod). That resulted in an obvious improvement in performance and the paper clip had moved, indicating proper actuation. I think I used too wide a tie wrap and too tight the first few times. Now have a smooth rush of power, with no noticeable bog. I did not remove the check ball, but I did enlarge the bypass groove slightly, as the factory groove looked very small and I wanted more vacuum signal to pass through. Thanks to patrickt and the rest of the commentators for this very informative and useful thread. I can now expect a reduction in the 13 mpg I have been getting up til now. This engine would probably be happier, and probably better performing with a 650 cfm carb. I'll save that for another time.