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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)



To replace the metering jets remove the metering rods and step-up springs as shown on the previous page. Next, disconnect the choke cam connector rod, pump connector rod. Finally, remove the 9 airhorn screws and remove the airhorn from the carburetor body.
 
A standard screwdriver can be used to remove the appropriate jets. Use the above table to select secondary main jets depending on desired level of fuel.
     


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.

 
While you have the top off it is a good idea to check the float adjustment. At free drop the float should be 1" from the underside of the top, as shown. You can carefully bend the attaching tabs to make adjustments.

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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