While the rod and jet control fuel flow, the metering rod spring controls the timing of the switch from cruise to power mode. A stiffer spring provides more tension against the vacuum attempting to lift the metering rod to the rich "power mode." Essentially, a stiff spring will open the richer fuel flow at a higher vacuum level, so the mixture goes rich sooner. Edelbrock has a spring assortment available to allow this transition point to be tuned. The base spring steps-up the metering at 5-inches Hg, and springs are available in steps from 3-8-inch Hg to alter the transition point. Tuning the spring tension is strictly related to driveability, since at WOT, any of the springs will have the same effect under near-zero vacuum conditions.

The spring comes into play if a flat spot (usually lean) is evident in the transition from light-throttle cruise to part-throttle acceleration. If such a flat spot exists in moderate part-throttle operation, checking the vacuum level with a vacuum gauge can help isolate the problem. Say, for instance, there is a flat spot occurring under moderate load at 6-7-inches vacuum, clearing up once the throttle is pushed a little harder, and the vacuum gauge reads below 5 inches of vacuum. If the stock 5-inch spring is still in place, it's safe to conclude that the flat spot is related to a lean mixture at the higher vacuum condition, resolving itself as the metering rod opens to the "power step" at 5 inches. Substituting a higher vacuum spring, such as one rated for 7 inches of vacuum, will likely provide the solution. As a rule of thumb, use the lowest vacuum-level spring that will provide smooth operation to prevent needless enrichment and loss of economy.

Secondary Metering
Calibrating the secondary side of the AFB-style carbs is simple in relation to the primary side. Charged only with providing mixture at high-throttle opening, a simple jet is all that is employed at this end of the carb. Tuning is simply a matter of swapping jets, the larger the richer, and the smaller providing a leaner mixture. Unlike the primary, where some range of adjustment is possible without disassembly of the carb via the metering rods, any change in the secondary jetting requires that the air horn be removed for a jet change. Changing secondary jets once the air horn is off is as simple as unscrewing the old ones and screwing in the new.

If a serious level of mixture calibration change is being performed on the carb, the primary and secondary should be adjusted to a similar, relative amount to arrive at the required final mixture setting. For instance, if the carb is found to respond to enrichment of the primary, the secondary should be enriched a like percentage. It's better to aim for an enrichment of 4 percent at both the primary and secondary circuits than to fatten the primary 8 percent and neglect the secondary. Edelbrock's handbook breaks down the percentage change for various jets and rods in their respective charts, providing an almost essential reference for the changes being made.