[SuperHart]

I agree with both of the above posts. Dyno values are obtained at WOT BUT most of our driving is not done at WOT. Changing the opening point by changing the spring should not affect dynoed performance unless you choose a heavy spring that prevents the secondaries from completely opening when needed. Considering that the purpose of vacuum secondary carbs is to open the secondaries only when the engine demand requires the extra air flow, tuning them with varying spring tension is a relatively easy task. If the secondaries open too quickly your engine will bog when you mash the throttle; too slowly and top end performance will suffer. Since your secondaries seem to be opening late you would want to begin your search with a spring one step LIGHTER than the black one. OH, by the way, don't judge the opening of the secondaries while you are parked and gunning the throttle. The engine must be operating under load for the secondaries to operate properly.

Keeping all that in mind and assuming that you want to tweak the secondaries, the Holley color coded spring kit will do the trick. Install your quick-change vacuum cover and refer to the color coded chart on the spring kit. Try a spring that is one step lighter than the black one in your carb, drive it for awhile and with several banzai attempts, some from standing launch and others while cruising. Keep trying lighter springs one step at a time until you hit one that causes the bog, then go back up one step heavier. That's how it is usually done.

Funny thing is, I did that on my dual quad 351W and ended up back at the stock springs. Oh well.