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.
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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. |
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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. |
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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.
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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. |
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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. |
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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. |
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 (Calibrating
Secondary Metering) |
| Membership Required |
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EPS
Model #1406
Rod & Jet Reference Chart
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| 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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