by FordMuscle Staff
Introduction
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.
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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. |
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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.
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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. |
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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.
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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. |
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We fired the car back up and with the car in Park, we
read an idle speed of around 830 RPM. |
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We gave the idle speed screw a slight turn clockwise to
bump the idle speed to exactly 850 RPM. A good starting
point. |
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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. |
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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. |
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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. |
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After adjusting both idle mixture screws our idle speed
increased from 850 RPM up to 900 RPM. |
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We adjusted the idle speed back down to 850 RPM. |
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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. |
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(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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