Fine-Tuning the Fuel System

Vintage Cars & Gasoline Part 2
Fine-Tuning the Fuel System
by Henry P. Olsen
Y
ou need to know the following air/fuel
mixture tuning techniques for carburetors if
you want to be able to get your customers’
vintage or muscle cars to perform their best on
today’s reformulated gasoline, but much of what we’re
presenting here will also work on any fuel-injected
(with a programmable computer) engine.
Gas is different
The formulation of gasoline today is very different
from the leaded gas of the past because of both political
concerns and environmental regulations. These
new reformulations are blended for the purposes of
producing the lowest possible evaporative emissions
and reducing the pollution from the exhaust. In many
parts of the country, oxygenates such as ethanol are
being added to gasoline in an attempt to reduce both
exhaust emissions and our dependence on foreign oil.
Anyone who is concerned about engine
performance and reliability needs to keep on top of
the ignition and air/fuel mixture tuning changes new
gasoline formulations require for maximum engine
efficiency. Reformulated gasoline has lower levels of
The author got the job of tuning
Eric Clapton’s street rod. We bet
you’d like to do that kind of work.
The Clapton car has a 650 cfm
Edelbrock Thunder carb.
sulfur, benzene, olefins, and reduced vapor pressure.
This affects jetting requirements because the burn
rate/characteristics and BTU content of the fuel is
different from what was available back when the car
was new. In short, reformulated fuel can negatively
effect the power, fuel efficiency and drivability of an
engine by 3 to 5% or more even when the engine has
the correct air/fuel mixture and ignition spark timing
for every driving condition.
tuning a carbureted-equipped engine, a 14.7
to 1 a/f mixture can be too lean because of the
operating losses in the induction system due to
intake runner and cylinder wall wetting. The
design of the intake manifold can also cause the
air/fuel mixture to vary from cylinder to cylinder.
If any one cylinder has an a/f mixture that’s too
lean, it will tend to misfire. Today’s reformulated
gasoline may require the use of slightly richer a/f
mixtures than were needed in the past.
The air/fuel mixture
The leaded gasoline that the engine in a classic or
muscle car was designed to use allowed it to perform
very well even if the spark timing or the air/fuel
mixture was not properly tuned. The same cannot
be said for the unleaded reformulated gasoline of
today. Unless these two factors are properly tuned,
the engine will not perform as it should.
The stoichiometric, or chemically ideal,
mixture of air and fuel for an engine running
on gasoline is 14.7 to 1 (14.7 lbs. of dry air to
one lb. of gasoline). In a perfect world where
an engine has this ideal air/fuel mixture, all the
energy gasoline and all the oxygen present in the
cylinder will be consumed during the combustion
process. The burn rate and energy content of this
one pound of gasoline, however, will change as
the formulation of the gasoline is changed (such
as with the addition of ethanol). When you’re
Proper tuning techniques got this ‘68
Charger to run just as well on today’s
reformulated gasoline as it did on oldfashioned leaded fuel back when the
car was new.
The Charger’s 440 breathes
through a Holley double-pumper
with an electric choke.
The chart above shows how the
Stoichiometric or chemically ideal air/fuel
mixture will change as the percentage of
ethanol in the gasoline is increased.
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Vintage Cars & Gasoline Part 2
Tools for reading the
air/fuel mixture
The ideal air/fuel ratio an engine needs to
produce maximum power and fuel economy, and
minimum exhaust emissions can best be calculated
in the laboratory with a dynamometer. Since we
are working with an engine that’s running on a
blend of gasoline that’s different from what it was
designed to run on, we have been using the readings
from a five-gas exhaust analyzer to determine what
air/fuel mixture the engine needs to have to run its
best with today’s gasoline.
In the past, tuning a fuel system was considered
by many to be a “black art” that seemed to be done
more by guess than by science. Back in the days of
leaded gasoline, an experienced mechanic could
attempt to tune the carburetor’s jetting by reading
the color of the spark plug nose. There were a few
very good tuners who could read a spark plug and
make the proper changes, but most of us were just
making an educated guess. Add in the unleaded
reformulated gasoline we have today, and the job of
reading of a spark plug is almost impossible -- little
or no color is left on the spark plug insulator. But
now you no longer have to guess what the air/fuel
mixture is because you can accurately read it with
an exhaust gas analyzer and/or a wide-band oxygen
sensor-based digital air/fuel meter.
This gas module and probe makes it
possible to read the content of the
exhaust on a scan tool while the car
is driven under real-world conditions
(courtesy OTC).
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Many automotive repair shops have a exhaust
gas analyzer (“smog machine”) that can be used to
read air/fuel mixture. Or, if you’re in a state such as
California that requires dyno-based smog testing, you
can read the air/fuel mixture under load. Using a dyno
to help in tuning is good, but we always like to read
air/fuel mixtures in real world driving conditions.
If you don’t have an exhaust gas analyzer, there are
several portable five-gas units on the market that are
quite affordable, or, for about $400, you can buy a
wideband oxygen sensor-based digital air/fuel meter.
These modern tuning tools can allow you to observe
what air/fuel mixture the engine is getting from the
fuel system while the car is being driven at any rpm or
load condition.
You should realize, however, that the readings you
get from a five-gas exhaust analyzer or a wide-band
oxygen sensor-based air/fuel meter are only as accurate
as the sample of exhaust is -- a smog pump or catalytic
converter will cause false a/f mixture readings. We
This relatively inexpensive a/f ratio meter
uses a wideband O2 sensor, and has the
advantage of real-time readings.
The exhaust analyzer also allows the part-throttle
air/fuel mixture to be checked, which is almost
impossible otherwise. It’s important to note that any
modifications other than jet changes and other basic
adjustments should be done by an expert to avoid
damaging the carburetor.
Targets
Starting points for air/fuel mixtures for most mild
performance engines are:
Idle:
•1.0% to 3.0 % CO, or a 14.1-13.4 to 1
air/fuel mixture
Cruise rpm:
This tailpipe probe allows you to
use a wideband sensor without
having to drill holes.
most often use a tailpipe probe to sample the exhaust
for the analyzer, or the exhaust clamp from Innovate
Motorsports so we can sample the exhaust without
drilling any holes in the exhaust system, but either
of these will have a dilution problem with pulsating
or low flow exhaust. When you want the very best
results, look at the content of the exhaust through
direct sampling. Sampling the exhaust from a direct
port in the exhaust pipe or header(s) gives readings
that can actually allow you to observe if you have any
fuel distribution problems that are causing the air/
fuel mixture to vary from cylinder to cylinder.
Tuning the a/f mix with a
five-gas analyzer
The easiest and most accurate method we’ve found
for tuning a carbureted engine’s air/fuel mixture is the
use of an infrared exhaust gas analyzer. Its readings
indicate the air/fuel mixture, the engine misfire rate
and combustion efficiency. The tuner can read what
the air/fuel mixture is at any operating condition, such
as idle, cruising speeds, or wide open throttle. The
HC, CO2 and NOx readings can be used to determine
if the ignition timing is correct for maximum power
and efficiency.
•1.0% CO, or a 14.1 to 1 air/fuel mixture with a mild
performance engine
•1.0% - 3.0% CO or a 14.1-13.4 to 1 air/fuel mixture
for an engine with a high performance cam
Note: A 0% CO or 14.7 to 1 cruise air/fuel mixture is
chemically ideal, but most carburetor-equipped
engines will suffer misfire and drivability problems
when operated that lean.
Power mixture and acceleration:
•6.6% CO or a 12.5 to 1 air/fuel mixture for a
production engine; a high performance engine
with improved combustion chamber design, such
as a Pro-Stock or a Winston Cup engine, in some
cases may be able to use a slightly leaner power
mixture of 4% CO or a 13.0 to 1 air/fuel ratio.
What the exhaust gas
content indicates
The readings from an infrared exhaust gas analyzer
will indicate air/fuel ratio, engine misfire, engine
combustion efficiency and excessive combustion
chamber heat (detonation) by looking at the following
exhaust gases:
•CO (carbon monoxide): This is one of the gases
we use to determine the air/fuel ratio. (Note: CO is
partially burned fuel.)
•HC (hydrocarbons): The amount of unburned
fuel, possibly an indicator of misfire. The best
mixture gives you the lowest HC.
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Vintage Cars & Gasoline Part 2
The above chart shows the gases in the exhaust that an infrared exhaust gas
analyzer reads and how the gases change as the air/fuel mixture changes. The 14.7
to 1 stoichiometric calculation assumes pure gasoline. Reformulated/oxygenated
gasoline needs a slightly richer ratio to be stoichiometric.
A hard seal kit will allow exhaust sampling upstream of the cat.
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•CO2 (carbon dioxide): The product of complete
combustion. The best air/fuel mixture gives you
the highest CO2 reading. The ideal ignition-timing
advance will also create the highest CO2 reading.
•O2 (oxygen): A high O2 reading indicates a lean
mixture, possibly due to an exhaust leak or a “hot”
cam (Note: when O2 readings are above 2 to 3%,
CO readings may be diluted by the extra O2).
•NOx (oxides of nitrogen): A gas created by
excessive combustion chamber heat. In many
cases, a high NOx reading is related to excessive
ignition spark advance. This causes high flame
temperatures too early in the combustion process.
This increase in NOx can be used as an indicator
of excessive combustion temperatures, which can
cause a ping or detonation problem that can lead
to engine damage.
Wideband tuning
The set-up that we use is the LM-1 Digital Air/
Fuel Meter from Innovate Motorsports. This unit can
allow you to sample and record the air/fuel mixture
data at a rate of 12 samples per second over a period
of up to 44 minutes. Then, you can confirm that the
tuning changes are giving the targeted fuel mixture.
Best to use both
You’ll get the best tuning results when you use both
the exhaust gas analyzer and the wideband oxygen
sensor methods. A wideband meter has the advantage
of being in real time, while an exhaust gas analyzer has
a response time delay of five to 10 seconds. The exhaust
gas analyzer, while slower, has the advantage that it
not only reads the oxygen/unburned combustibles,
but also allows you to determine:
•the air/fuel mixture by observing the CO reading.
•the engine’s rate of misfire by observing the
HC reading.
The air/fuel mixture can also be checked with a
wideband oxygen sensor installed in the exhaust
system. The sensor signal is then read by a digital
air/fuel meter. This method determines the air/
fuel mixture by looking at the oxygen/unburned
combustibles in the exhaust. The readings can be
very accurate, but false readings can be created by
anything would changes the oxygen or unburned
combustible content of the exhaust. These false
readings are created when the sensor “sees” the
excessive oxygen that can come from a misfire,
a supercharged engine’s “blow-through” effect
(at lower engine speeds), a high performance
camshaft’s valve overlap, or an exhaust leak.
Performance carbs
Using a wideband oxygen sensor-based air/fuel
meter system requires that you know what air/fuel
mixture your engine needs for each driving condition.
This data should be available from your engine builder,
or you can use an infrared exhaust gas analyzer to
help you determine what air/fuel mixture allows your
engine to run at its best. An extended range oxygen
sensor can read air/fuel mixtures as rich as 9 to 1
and as lean as 19 to 1 (the traditional switching-type
oxygen sensor is only accurate at air/fuel mixtures of
around 14.7 to 1). This method has the advantage of
extremely fast reaction times for the readings, but
the accuracy of the reading can be affected by engine
misfire, detonation, or exhaust sample dilution.
While original equipment carburetors can be tuned
to run quite well, many of our customers want a
Holley, Edelbrock or Barry Grant for their cars. Each
of these replacement performance carburetor designs
has advantages and disadvantages when compared
to the others on the market. The Carter-designed
AFB and AVS carburetors sold today by Edelbrock
as Performer and Thunder carburetors are good, but
they’re not the best for a customer who likes to drive
his or her car fast around corners (because of the main
jet placement and the float bowl design). The Holley
and Barry Grant Modular designs are very good, but
they do tend to require more maintenance than most
others. Our personal preferences dictate that when
•the engine’s
CO2 reading.
efficiency
by
observing
the
•spark knock, detonation or engine ping, which is
caused in most cases by overly advanced ignition
timing, by observing the NOx reading.
The spark plug color tuning method of days past
would only indicate what the average air/fuel mixture
was just before you pulled the spark plug.
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Vintage Cars & Gasoline Part 2
a customer wants a carburetor with an automatic
choke, we recommend a Holley, but when horsepower
is the goal, we recommend a Mighty Demon carb
from Barry Grant.
The Mighty Demon is great from a tuner’s
viewpoint because it has changeable idle-feed
restrictor/jets, plus changeable idle and main well
air bleeds. These carburetors work quite well with
the high-performance air-gap intake manifolds that
are becoming very popular. These manifolds don’t
use an exhaust crossover, so there’s no
heat to help keep the fuel atomized as it
passes through the intake ports on its way
to the cylinders. With an air-gap intake
manifold, the Mighty Demon does a better
job supplying the richer mixtures the
engine needs during idle and part throttle
modes without having to modify the idle
system than any other high performance
carburetor we have worked with.
Fuel Delivery Circuits
A carburetor has fuel supply circuits for
idle, off idle, cruise/main metering, power
and acceleration that are designed to provide
the air/fuel mixture the engine needs during
all operating conditions. Each of these
circuits may need tuning attention.
Above: The Holley double-pumper was
one of the first high-performance
carburetors, and can still hold its own.
Above: The Carter-designed Edelbrock
Thunder is an easy-to-tune carburetor for
a mild performance engine.
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The idle system of most carburetors has an idle
jet/restriction, an ICR (idle channel restriction)
and air bleeds that supply the air/fuel mixture to
the idle mixture screw(s) and the offidle transition ports. The ICR limits the
air/fuel mixture that is delivered by the
off-idle transition ports. These ports
provide fuel from the point when the
throttle is first opened to when it’s open
far enough for the main jets to take over
(about 2,500-3,500 rpm). The metering
block of a modular-style carburetor such
as a Holley model 4150 does not use an
ICR because the diameter and taper of
the idle well serves the same function.
If the air/fuel mixture is too lean at
idle and the idle mixture screws do not
provide enough adjustment, the idle jet/
restriction may need to be enlarged. If
the air/fuel mixture is correct at idle, but
lean at 1,000 through 2,500 rpm, the ICR
may have to be slightly enlarged to allow more fuel
to be delivered at part throttle. A lean condition at
Carter-designed AFB (Edelbrock Performer) and
AVS (Edelbrock Thunder) carburetors often have
lean off-idle stumble problems that can be cured
by enlarging the ICR on the 500 through 650 CFM
units. But this cure often doesn’t work with the 750
and 800 CFM models. Many of the Holley modular
carburetors we have tuned have also had some
lean off-idle stumble problems, which we fixed by
enlarging the idle well in the primary metering block.
Tuning cruise and
power mixtures
Above: A Mighty Demon on a
500+ hp small-block that powers
a streetable ‘69 Camaro.
The power enrichment system used in most
domestic carburetors can be either a power valve
that’s held closed until engine vacuum drops below
its opening point (at which time it richens the air/
fuel mixture), or a vacuum-controlled power piston
system that moves tapered metering rods out of the
main jets, thus causing the air/fuel mixture to become
richer as the vacuum drops.
If the engine you are working on is equipped with
a power valve, the main jet size is what determines
what air/fuel mixture is delivered to the engine at light
load/cruise speeds, and the power valve restriction
(under the power valve) is the determining factor
in how richer the air/fuel mixture will be when the
power valve is open. A 6.5 “inch” power valve will be
open and supplying the richer air/fuel mixture needed
under high power demands anytime the vacuum is
below its 6.5 in. Hg opening point.
Above: The arrows point to the
changeable air bleeds that make the
Mighty Demon tuner-friendly.
part throttle will cause the engine to miss or stumble
on light acceleration and 15 to 30 mph light cruise
speeds. This lean problem is very common on many
original carburetors and most of the performance
replacement carburetors we see today. If the air/fuel
mixture is too rich at idle and/or part throttle, the idle
jet or part throttle idle restriction may be too big and
may need to be replaced with a smaller one. Or, you
could enlarge the idle system air bleeds.
On the metering rod type, a power piston with a
spring under it controls the rods. When vacuum
drops below the spring’s opening point, the metering
rods will rise, thus effectively enlarging the jet
and supplying the richer mixture the power mode
requires. You can change the power or cruise mixture
by changing the jets, metering rods, or a combination
of both.
Tuning the power system opening point can also
be done to increase gas mileage. If you make it open
at a lower vacuum, it will take more load or throttle
opening to get the richer power mixture to kick in. You
can change to a power valve that opens at 4.5 inches of
vacuum instead of 8.5 inches. With metering rods, the
opening point can be modified by changing the power
piston spring.
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Vintage Cars & Gasoline Part 2
The fuel economy an engine gets will be its best
when the air/fuel mixture is at or near 14.7 to 1 (with
non-reformulated gasoline), but many carbureted
engines don’t perform very well with that lean a blend.
We use a five-gas exhaust gas analyzer to find what
the ideal mixture is for fuel efficiency. At the ideal a/f
mixture, all the O2 in the cylinder will be burned, the
lowest amount of engine misfire as indicated by HC
will occur, and engine efficiency as shown by CO2 will
be at its highest. Experience has shown that the best
a/f mixture for gas mileage is just slightly on the rich
side of the point where the misfire rate starts to rise
from a mix that’s too lean to burn.
Below: Hesitation with an Edelbrock carb
may be due to a weak accelerator pump
spring (right). The Ole’s replacement
(left) duplicates the duration of the
original Carter unit (center).
Tuning the accelerator pump
Tuning the accelerator pump squirt volume and
duration is often done on a trial and error basis to
obtain the best throttle response, but a 12.5 to 1 air/
fuel mixture is a good place to start. One of the most
common complaints we hear from our customers is
hesitation on quick acceleration, which is common
with many aftermarket performance carburetors. Due
to cost-cutting procedures and changes in production
materials the accelerator pump duration spring is not
as strong as the original that was used when these
carburetors were first in production. The cure for
Edelbrock Thunder and Performer carburetors is an
accelerator pump with a stronger duration spring
(Ole’s part #1010). When we’re working on a Holley
modular carburetor, we use an accelerator pump
upgrade kit that has a stronger duration spring, a
0.031 squirter and a “pink” cam (Ole’s part #1330)
that makes the accelerator pump more active.
Road test
Once you’ve gotten the idle, part throttle and cruise
air/fuel mixture curves correct, the next step is a road
Below: This accelerator pump
upgrade kit for Holleys boosts
squirt to cure tip-in hesitation.
Below: The ‘60s-era Holley pump arm
on the left has a stronger spring than
the new-style arm on the right. A
replacement spring that duplicates
the early version is available.
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This should give you an idea of how
much gasoline can run through a
carburetor with the pedal to the metal.
1/4 Mile E.T.
Time to Fill a
1-Gallon Gas Can
7 sec.
Under 12 seconds
8 sec.
15 sec
9 sec.
20 sec.
10 sec.
25 sec
11 sec.
30 sec.
12 sec.
35 sec.
test using a portable infrared exhaust gas analyzer and/
or a wideband oxygen sensor. This will allow you to
check the cruise speed air/fuel mixture, followed by a
check of the power mode a/f ratio under load. You will
be able to see what the mixture is during real-world
driving conditions, then correct the mixture. A chassis
dyno can allow you to tune for a loaded condition, but
the airflow conditions into the carburetor on the road
can cause the mixtures to change.
If a reading goes too lean at high engine loads, the
first thing to do before you change jet size is to check
that the fuel pressure. It must stay above five psi at
wide-open throttle or the carburetor will starve for
fuel. Low fuel pressure could be caused by the fuel
pump not having enough volume for the engine’s
needs, or the fuel tank vent system may not be letting
enough air into the tank to replace the fuel being
removed from it. Many replacement gas caps do not
vent the gas tank as well as the original did, so you
may need to add extra venting to the gas tank.
Once the ignition advance and air/fuel mixture
curves are tuned for today’s unleaded reformulated
gasoline, the engine should perform as well, or maybe
even better, than when the vehicle was new, and you
will have a very happy customer!
Believe it or not, a high-performance engine can actually drain its carb bowl during
hard acceleration, so you have to watch fuel pressure.
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Vintage Cars & Gasoline Part 2
Resources:
•Barry Grant, Inc.
www.barrygrant.com
•Bridge Analyzers (exhaust gas analyzers)
www.bridgeanalyzers.com 1-510-337-1605
•Edelbrock Corporation
www.edelbrock.com
•Holley Performance Products
www.holley.com
•Innovate Motorsports
(digital air/fuel meters)
www.innovatemotorsports.com
1-949-388-4442
•Ole’s Auto Parts
(fuel system and ignition parts)
www.olescarb.com 1-650-589-7377
•OTC/SPX Corporation
(exhaust gas analyzers)
www.otctools.com 1-800-533-6127
•Renewable Fuels Association
www.ethanolrfa.org/resource/technicians/
•Rockett Brand Racing Fuel
www.rockettbrand.com 1-800-345-0076,
E-mail questions to: [email protected]
A 650 cfm Mighty Demon on a high-performance crate motor makes a potent
combination -- providing it’s tuned right.
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