Lets go over the different adjustments and what they do.
Here's a governor (aka slot plate) installed with its springs and weights.
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1. RPM advance begins depends on the initial spring force on the wieght.
That
spring force holding the wieght back depends on three things.
a. Spring rate.
b. Length spring (loop to loop).
c. How much the spring is stretched.
With a light primary spring like the factory used, a small change in stretch caused by welding the inside of the slots can easily be compensated for by adjusting the spring perch.
Factory primary springs tend to be about .75" loop to loop. Obviously they vary.
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Lets compare with some other springs
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The Gardner springs in the middle are about the same size but made of heavier wire.
They are close enough in length that with a little bending of the loops and the initial spring force can the be fine tuned with the perches so the advance starts immediately above idle rpm.
That's why in the distributor shown above a heavy primary spring can be used.
2. Advance rate above idle.
This is entirely dependent on the primary spring. If we want a faster or slower advance, the spring needs to be changed.
3. Shaping the advance at mid to high rpm.
The second weight moves out with the primary wieght because its locked into the slots. When its moved out far enough, the second spring joins in. Moving the secondary spring perch, or using a secondary spring with a different length loop changes the degrees of advance before the heavy spring applies force.
You can eyeball how many degrees into the advance before it contacts like I'm doing here with a tach drive distributor.
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To bring the second stage in earlier, the tab type perch would be bent a little outward.
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