Calibrating Part-Throttle
The usual indication that the part-throttle calibration needs adjustment is surging while cruising at steady cruising speeds or flat spots on light accelerations. These symptoms are generally a sign of a mixture that is too lean. The cure is to go slightly richer in the cruise mode; select a metering or a metering rod and jet combination that results in a richer cruise mixture while not changing the power mode.
Edelbrock calibration kits come with a selection of jets, metering rods and step-up springs. When changing calibration, always try to swap rods only. It's quick and easy. | | |
Choosing Step-Up Springs
The metering rods and springs determine power modes, but when the metering rods go from one mode to the other is determined by the step-up springs. The base calibration uses springs that stage, or allow the mixture to become rich, when the manifold vacuum drops to 5 in-Hg. Springs in the calibration kit are color-coded; blue, 3 in-Hg; yellow, 4; orange, 5; pink, 7; silver, 8.
If as the throttle is opened gradually the engine stumbles slightly then clears up, it may be a staging issue and a stronger spring will help. On our truck a surging condition during moderate acceleration was cured with a lighter spring.
Here one of the step-up springs can be seen. The springs push the pistons up and manifold vacuum pulls them down. The springs determine when the metering rods change from cruise to power and back. | 
With the covers off, the tops of the vacuum pistons can be seen. | 
The staging springs fit in wells below the vacuum pistons. While simple, they perform a vital function. |