From the CARDFORD site. So, which is it? Replace the heavy one or the light one. I've seen it both ways. I can see replacing the light one to bring advance sooner and bending the heavy tab to bring in the total sooner.
Standard distributors have the rate of advance controlled by two springs attached to the centrifugal weights. The two springs are usually of different tension and length. Frequently the heavier of the two springs is not actually under any tension when the distributor is stationary, however the weaker of the two should have absolutely no slack when stationary. If the lighter of these two springs does have some slack, it is important to take this up so that the spring is under slight tension otherwise the timing settings will be altered immediately the engine turns over. If there is no tension in the spring, bend the stop on that spring to re-instate tension.
In most applications for performance engines, the heavier of the two distributor springs requires replacement with a lighter spring. Alternative springs can be gleaned from a scrap distributor (it does not need to be of the same type necessarily) from a breakers yard. This is necessary to give a faster advance ramp, as seen in the previous table.
When the heavy spring is replaced by a lighter one, BOTH springs must be under tension, but not under too much, there must be no loose rotational movement between the central shaft and the weights as this will give too fast a ramp and lead to pinking. The static timing, if set as before is plenty, and we do not want any more until around 1300-1500RPM. If the distributor does not have bendable posts that allow the spring tension to be varied, then the springs may need to be tweaked a little to shorten or lengthen them.