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The power piston meters additional fuel by lowering and raising the primary metering rods in and out of the jets. The purpose of the power piston spring is to raise up the power piston and flow more fuel when the engine vacuum gets low enough (under higher load conditions). Taking into account our mild cam specs, Sean selected a purple spring which comes on at 9 in-Hg, and is fully in at 5 in-Hg. A wilder-cammed engine would be more suited to a lighter spring rate to compensate for producing less vacuum.