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Tuning Part Throttle Driveability
Part throttle driving consists of a "cruise" mode, where there is steady throttle and light acceleration. In this mode engine vaccum is high. Part-throttle "power" mode when the engine is under greater load, but not a wide open throttle. Engine vaccuum is lower than cruise mode, but not zero as in wide open throttle. The design of the EPS carburetor allows adjustment of both part throttle modes by calibrating the Primary Metering. Unlike other carburetor designs, in an Edelbrock carburetor this can be achieved without affecting Secondary Metering, or the WOT system.

The primary metering system consists of a intricate mechanism of a jet, metering rod, and step-up spring. The step-up spring sits in a piston within a vacuum referenced passage in the carburetor body. See Figure 1. Manifold vacuum creates a suction under the piston which compresses the spring and in turn seats the tapered end of the rod into the jet orifice. This creates a restriction to fuel passage and thus a leaner air-fuel mixture. Under load, when manifold vacuum is low, the spring tension forces the piston and rod upwards, moving the tapered end progressively out of the orifice, creating a larger passage for fuel flow and thus a richer air-fuel ratio. It is clear to see then how the use of different sized rods and jets can change the amount of fuel delivered, and the use of heavier or lighter springs can effect at what vaccuum the fuel is delivered.

Most part throttle drivability problems with an Edelbrock carburetor are attributed to poor primary metering calibration. Without an air-fuel meter, dialing-in Primary metering involves driving at a variety of engine speeds while paying special attention to "flat spots", "stumbles", and "surges". It is also helpful to have a vacuum gauge in the car to see if the problems occur at part-throttle cruise or in the power mode, where you might be rolling into the throttle and accelerating with low vacuum levels.
Depending on how your car is reacting you will change metering rods, jets, and springs to eliminate trouble spots. Edelbrock supplies a calibration reference chart with all of their carburetors. We used model EPS 1406 for this article. See Figure 2.



Reading the Calibration Chart
First, find the number corresponding to your current rod and jet setting. The center position number 1 is the factory setting. Next, identify whether you are interested in adjustments to Cruise mode (vertical axis) or Power mode (horizontal axis.) Finally, move in the desired direction, lean or rich. For instance, if you are at the factory setting, number 1, and you require one stage richer in the cruise mode, then you are at marker 24. Refer to 24 on the lookup table for the jet and/or rod combination to change to. Black markers mean both a jet and rod change, while white is only a rod change. Keep in mind we we're working with an EPS 1406. The Calibration Reference Chart is specific to the EPS model. They are available online or in the orginal tuning book supplied with the carburetor.



When you order your Edelbrock carburetor you shouldn't think twice about the calibration kit, this is part number 1487 for the 1406 600cfm carb. It includes an assortment of rods, jets, and springs.
 
Shown are typical metering rods for the EPS carburetor. They are identified with two numbers, such as 0.068 x 0.047. This refers to the dimensions of the tapered areas at the bottom of the metering rod.
     

The step-up springs are color coded by the vaccum level at which they compress. The base spring (orange) is rated at 5" Hg. Blue = 3", Yellow = 4", Pink = 7" and Plain = 8"
The stronger the spring the quicker the rod will move out of the jet and allow fuel to pass through. Therefore if you are experiencing a stumble as you first press the accelerator try the next stiffer spring. You can also use a vaccum guage and note the reading when you begin to accelerate. Select a spring 1" greater than the lowest reading you observe.
 
Edelbrock main jets are used in the primary and secondary sides of the bowl. They are marked with a "120-###" The last three digits minus 300 gives the actual orifice size in thousandths. For example, a 120-389 is a 0.089" jet. Unlike Holley carburetors it is common for EPS carbs to have larger primary jets than in the secondary due to the metering rod restriction.
     

To change out the primary rod, jet or spring back out the torx head screw holding the cover plate. There is one plate for each side and the screw need not be removed completely as the plate will swing outward to reveal the metering rod. With the plate moved over the piston is likely to pop up due to the step-up spring pressure. Be careful so as not to lose the spring.
 
Shown is the actual metering rod, jet, piston and spring. The rod is held through the piston head with a small clip, always remove this clip away from the motor, preferably on a clean flat work surface to avoid losing it. The entire assembly, minus the jet, comes out as a unit.
     
(Calibrating Secondary Metering)
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EPS Model #1406
Rod & Jet Reference Chart
No.
Main
Jet
Metering
Rod
Change From Base
1
1427
(.098)
1459
(075x047)
stock
2
1426
(.095)
1449
(070x037)
Rod & Jet
3
1427
(.098)
1458
(075x037)
Rod
4
1425
(.092)
1446
(068x042)
Rod & Jet
5
1426
(.095)
1453
(071x047)
Rod & Jet
6
1425
(.092)
1447
(068x047)
Rod & Jet
7
1426
(.095)
1456
(073x042)
Rod & Jet
8
1426
(.095)
1455
(073x042)
Rod & Jet
9
1426
(.095)
1454
(073x037)
Rod & Jet
10
1425
(.092)
1451
(070x047)
Rod & Jet
11
1425
(.092)
1453
(071x047)
Rod & Jet
12
1426
(.095)
1459
(075x047)
Jet
13
1425
(.092)
1449
(070x037)
Rod & Jet
14
1426
(.095)
1458
(075x037)
Rod & Jet
15
1426
(.095)
1448
(068x052)
Rod & Jet
16
1426
(.095)
1447
(068x047)
Rod & Jet
17
1427
(.098)
1453
(071x047)
Rod
18
1429
(.101)
1459
(075x047)
Jet
19
1427
(.098)
1449
(070x037)
Rod
20
1425
(.092)
1445
(065x047)
Rod & Jet
21
1426
(.095)
1451
(070x047)
Rod & Jet
22
1426
(.095)
1446
(068x042)
Rod & Jet
23
1427
(.098)
1455
(073x042)
Rod
24
1427
(.098)
1456
(073x047)
Rod
25
1427
(.098)
1454
(073x037)
Rod


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