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by FordMuscle Staff

Someone once said that a carburetor is an incredibly ingenious and complex device for providing the wrong air-fuel ratio under all conditions of load and speed. Since the first carburetor design by Hungarian engineer Donat Banki in 1893, countless carburetor manufacturers have worked towards innovative designs that prove just the opposite - that a carburetor is actually a very reliable and precise fuel metering device. It perhaps could be argued that this objective was never really fulfilled, considering the replacement of carburetors with fuel-injection in factory cars since the mid 1980’s. Or you could take the glass is half-full view and point to the continuing engineering and advancements being made in carburetion by companies such as Holley, Barry-Grant, Edelbrock, and others. Let’s face it, carburetors will be in demand so long as guys are into the cars that were originally equipped with them. Carburetors are relatively cheap and simple to tune, and when it comes to wide-open-throttle (WOT) performance they match, or in many cases, out-perform EFI systems. Does this mean that carbs are only best suited for maximum performance use, where fuel economy and idle or part-throttle driveability is of no concern? Absolutely not, but it does depend on which carburetor you select.

For the enthusiast who wants hassle-free carbureted driving, while ensuring his venturis aren’t needlessly sucking down todays high priced gasolines, the Edelbrock Performer Series (EPS) carburetors have an outstanding reputation. Edelbrock completely redesigned and retooled the original Carter design to produce a carb that offers near-EFI streetability and economy while retaining good WOT performance. Even if you are not likely to find them under the hoods of race cars at the drag strip or track (case in point, check what is under the hood of Edelbrock’s 66 Shelby Vintage Racers), you certainly will also not find them under the hood of any motorist stranded along the highway with carb problems.

Using an air-fuel meter is the preferred way to tune, but the EPS carbs can also be successfully tuned with just conventional tools.
Proper Tuning
As with all carburetors the Edelbrock Performer Series (EPS) carburetors require proper setup and tuning to achieve trouble free driveability and performance. The best way to tune any engine, carburated or fuel injected, is by using a wide-band oxygen sensor. By being able to see the air-fuel ratios throughout the drive cycle one can precisely adjust the carburetor to add or take out fuel. We have quickly become fans of the Innovate LM-1 wide-band O2 sensor and data acquisition tool for our performance oriented project cars. However at about $500 for the kit it is still not likely to be in everyones tool box. Therefore, as hard as it was, when it came time to tune the 600cfm EPS carb on our Project MX, we left the LM-1 at home, to show that the EPS carburetor is easy to tune the old fashioned way - with just your senses, a tach and a vacuum gauge. It's been on for several months now, and after having dialed-in the idle, primary and secondary circuits, there is not the slightest bog, stumble or surge, even at altitude here in Nevada. Follow along as we explain how these carburetors work and show you how to properly tune them for economy, drivability, power, and emissions.

Setting Idle Mixture
The idle mixture setting, which we will demonstrate, is critical both to a smooth idle at proper RPM and to a smooth transition to part throttle operation. The transition from idle to part-throttle is essentially the hand-off of the Idle System to the Primary Main System. Calibrating idle mixture on an Edelbrock carburetor is controlled by equally adjusting the two screws at the front of the carburetor. Turning them out creates a richer mixture and screwing them in makes the idle air-fuel ratio leaner. The goal with any idle setting is a "lean-best" setting - that is, the leanest possible ratio yeilds the maximum idle vacuum while maintaining the rpm set point. Here is how to do it.

Prior to beginning any tuning we made sure the choke was set properly. It should be connected to a 12V key-on source. With a cold motor and the choke flap closed there should be no less than .100" clearance as shown. Other things to check are fuel pressure of no more than 6psi and proper float adjustment.
For the purposes of setting idle mixture we used a tach/dwell meter, spare Autometer vacuum gauge, and standard screwdriver. The Edelbrock EPS manual is a great reference and the rod and jet kit will be used for the primary and secondary tuning later in this article.

Setting idle mixture should be done with the engine at full operating temperature (choke fully open) and the car in Park, or neutral for manuals. Prior to starting up the car we gently seated the idle mixture screws and then backed them out 2 full turns. We were confident this would make our idle mixture slighty rich to start.
At this point, we fired up the engine and let it get up to operating temperature (thermostat open). Once the car reached operating temperature, we shut it down.

Next, we connected our tach/dwell meter. Typically, one lead connects to the negative side of the coil while the other attaches to a good ground. Alternatively you can have a helper read off the rpms from a dash mount tach.
We hooked up our vacuum gauge to manifold vacuum and set the gauge in a safe spot where it couldn't roll into the fan or on to a hot exhaust header.

We fired the car back up and with the car in Park, we read an idle speed of around 830 RPM.
We gave the idle speed screw a slight turn clockwise to bump the idle speed to exactly 850 RPM. A good starting point.

Next, we took note of our vacuum reading before we began adjusting the idle mixture screws. At 2 turns out we had just under 15" Hg.
Confident that our idle mixture was running a bit rich, we turned one idle mixture screw in a half turn and watched for a rise in vacuum.

As we expected, we got a slight rise in vacuum up to almost 16" Hg. We gave the same screw a lean bump and watched for more vacuum. Once we determined that leaning the mixture further was not increasing vacuum, we turned the other idle mixture screw in the same amount.
After adjusting both idle mixture screws our idle speed increased from 850 RPM up to 900 RPM.

We adjusted the idle speed back down to 850 RPM.
Finally, we noted the effect the corrected idle speed had on our vacuum reading. Looked good. Remember, the goal of adjusting idle speed is to achieve the highest idle vacuum you can at a set idle speed.
(Calibrating Primary Metering)
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In This Article:
Take the complexity out of tuning your Edelbrock Performer Series carburetor in this straight-forward tutorial that will exorcise your carburetor's demons.



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