Tuning for Wide Open Throttle
When the primary throttle blades
have opened 65% the secondary throttle blades begin to open.
They open at a faster rate such that both primary and secondary
throttle blades reach full vertical (WOT) at the same time.
Edelbrock carburetors also utilize two devices positioned
above the secondary throttle blades called Air Valves. See
Figure 2. The air valves are counter balanced with a weight
and open via air velocity. The point of the air valves is
to create a low pressure zone which pulls additional fuel
out of a discharge nozzle placed in the body of the carburetor
near the edge of the air valve when it is closed. The air
valve system functions as an auxillary fuel addition method
during the transition to WOT to prevent a lean bog. When the
air velocity reaches a certain point the air valve is vertical
with the secondary throttle blade and fuel is added only through
the secondary main jet system.
Tuning for WOT is best done at the track or on the dyno, where
you can use objective performance results to assess the impact
of tuning changes. Even here using an air-fuel meter alone
is not a  guarantee
of maximum performance, though it will aid in obtaining a
safe air-fuel ratio. Because WOT causes the most stress on
an engine we want to target for a safe air-fuel ratio that
biases the rich side. With an air-fuel meter this would be
roughly 12.2-12.7:1 for a naturally-aspirated engine using
unleaded fuel. Without an air-fuel meter we can tune for this
by driving the car at WOT in third or fourth gear through
the entire power range (for most street cars this would be
2500 to 5500 rpm.) At the end of the run, shut the engine
off and stop the car without letting the engine idle. Pull
a spark plug on each cylinder bank and check the insulator
color. A light tan to brown color indicates the desired air-fuel
ratio. An insulator that is bone white or white with black
specs indicates a lean condition and you should jet the secondary
side up as shown below. Note that reading plugs can be difficult
in states such as California where fuel additives result in
a very clean, no-deposit, combustion. In these cases you will
want to use your senses to detect possible detonation.
The following chart (specific to the #1406 600cfm carburetor)
shows the secondary main jet changes to make in order to go
4, 8, or 12% richer or leaner than the factory setting. Similar
tables can be found for other part number EPS carburetors
in their respective manuals.
3 Stages
Lean (12%) |
2
Stages
Lean (8%) |
1
Stage
Lean (4%) |
SECONDARY
Stock Calibration |
1
Stage
Rich (4%) |
2
Stages
Rich (8%) |
3
Stages
Rich (12%)
|
Jet
#1423
(.086) |
Jet
#1424
(.089) |
Jet
#1425
(.092) |
Jet
#1426
(.095) |
Jet
#1427
(.098) |
Jet
#1429
(.101) |
Jet
#1430
(.104)
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Conclusion
 Hands-on
automotive enthusiasts typically strive for a perfect carburetor
tune to achieve perfect drivability and performance. A welcome
by-product of a properly calibrated EPS carb is the word "PASS"
printed in the Hydrocarbon (HC) and Carbon Monoxide (CO) columns
of an emissions report. An EPS carburetor running with a rich
idle mixture or with a rich part-throttle rod and jet combination
will prevent the freshest motor from passing a smog check.
The real-world validation of the tuning procedures outlined
in this article came during a smog check in May of this year
when we were registering Project MX. The Nevada Divsion of
Environmental Protection requires that non-exempt passenger
cars built before 1980 emit less than 700 ppm HC and CO levels
must not exceed 3.5%. Measurements are taken at 1000 and 2500
RPMs. The accompanying photo tells the story. 
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Posted by dizerega2, 04/03/07 02:54pm:
Well written and informative--a good addition to the Eldebrock carb manual.
Posted by EBCobra, 07/10/07 12:55am:
A great road map for tuning the Brock carb.
Posted by ourayrider, 05/22/08 06:58pm:
This article was much more helpful than others I had read. It had just what I was looking for. I'm anxious to try it out.
Posted by csharp, 06/23/08 06:54pm:
Good article, however the base spring on a 1406 carb is yellow, not the orange.
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As with most carburetor designs the EPS also utilizes
a pump-shot mechanism to squirt fuel into the primary
side of the carburetor during rapid opening of the throttle
blades. This is to prevent a lean condition that occurs
as a result of the air moving faster than the fuel.
The amount of fuel delivered via the pump shot is adjusted
by changing the level position, moving to a hole closer
to the body extends the stroke. 
