I've had all of my WCFB's apart. Some had the springs under the accelerator pump and others did not. The spring under the power piston indeed does hold the metering rods in the Jets and for a small amount off ez driving idle; but there is an arm that you set with a screwdriver the forces the metering rod actuating level up under more throttle no mater what the vacuum. I believe the vacuum helps pull the metering rods back down more than any thing. I've tried minor adjustments in the setting of the adjusting arm to improve fuel milage but they seem to be the best as the factory sets them. There are different stepped metering rods too.I like them anyway and am going to keep them... Good Luck.