That spring controls the rate (not the amount) of vacuum advance. A stronger spring delays the point at which the ported vacuum overcomes the spring tension and allows full advance during light throttle cruise conditions. As soon as you step into the throttle a tiny bit, all that advance goes away until you return to light throttle cruise. If you have a chassis dyno and a few thousand dollars' worth of other test equipment, it would be possible to wring out the last few fractions of gas mileage and performance by tinkering with the spring tension. In practical terms, buy a handful of slightly different springs, and spend several months keeping very meticulous records on fuel consumption, uphill performance on the exact same road for dozens or runs, etc. Be sure to compensate for variations in temperature, atmospheric pressure, relative humidity, and about half a dozen other variables. That's how the researchers who designed the thing in the first place did it. Have fun, and be sure to publish your findings here so we can argue about them!
Jerry
