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Edelbrock Performer Series Carburetor Owner’s Manual 8/94
The fuel flow rate in the Main System is proportional to the air flow rate; as air flow increases - from
either an increase in throttle opening or an increase in engine speed at the same throttle opening -
the fuel flow also increases by nearly the same degree.
At higher engine loads, such as in a heavy part-throttle acceleration, there is a need for a richer
mixture. This enrichment is provided by the Metering Rod and Step-Up Function (Figure 4). A vacuum
passage (8) communicates the manifold vacuum to the underside of the Step-Up Piston (9). This
vacuum tries to hold the Piston in the bottom of its bore by working against the force of the Step-Up
Spring (10).
When the manifold vacuum is high, indicating a low load such as idle, cruise, or light acceleration, it
is able to overcome the force of the Step-Up Spring and hold the Step-Up Piston at the bottom of its
bore, which also positions the Metering Rod at the bottom of its travel. At this point, the Rod has a
large diameter that creates a high restriction through the Jet and the fairly lean A/F Ratio that is
desirable for low load/low power operation. This portion of the Metering Rod is referred to as the
“Lean Step” of the Rod.
When the manifold vacuum is low, indicating a high load such as a heavy part-throttle (or WOT)
acceleration, the Step-Up Spring is able to force the Piston to the top of its bore and position the
Metering Rod at the top of its travel. This action is called “Power Mode Staging”. The portion of the
rod now located in the jet has a smaller diameter, thus the restriction through the Jet is reduced and a
rich A/F Ratio is provided for high load/high power operating conditions. This is the “Rich Step” of the
Rod.
Metering Rod and Step-Up Function
Figure 4
Step-Up Piston
Step-Up Spring
Metering Rod
Main Jet
LOW LOAD: High Vacuum ROD UP: Rich A/F Ratio
ROD DOWN: Lean A/F Ratio HIGH LOAD: Low Vacuum
LEAN STEP In Jet
RICH STEP In Jet
1403-1413 Perf Series Carburetor Owner's Manual.qxd 3/31/2006 12:10 PM Page 5