...On All My Vacuum Secondaries 600-750, I had the best results using the SHORT BLACK Spring (NOT the LONGER Black spring), shown in the Photo. Regardless of the Chart...It seems to be in the mid-range of that Spring Set, Average. Not too Weak, Not too Strong. The Secs will open-up right along with the Engine Revs. Quickly and Balanced. NO Bog, No Stutter, No Hesitation. Strong!! FWIW
A good place to start your testing. And, Be Sure the Accelerator Pump is Adjusted Precisely. As in the AP Cam is on the correct hole, and more.
MJ