Car Craft July 2005
You’ll Need Some Electronics
to Do It, but Now It’s Easier Than Ever to Be A Carb-Tuning
By Jeff Smith
In a time not long past there was that guy down the street from
you – that special car-crafting guru who could tune an engine by
ear. He was revered by the local gearheads for his uncanny knack
for creating not just impressive power, but also transforming chugging
street slugs into razor-sharp sweetheart engines. He was like the
Pinball Wizard – the deaf, dumb, and blind kid who could play those
flipper fingers like he was part of the machine. We were all in
awe of the Carb Wizard. While that guy still exists, we’ve discovered
a little piece of electronic technology that can turn almost any
knowledgeable car crafter in a Carb Wizard. This latest development
in hand held electronic-power knowledge is the affordable air/fuel-ratio
meter. While there are many on the market, we first ftold you about
a favorite, the Innovate Motorsports digital unit, in “Tune In,
Turn On, and Make Power” (Feb. ’04) where we explain how it works
alongside a few basics concerning air/fuel ratio and horsepower.
This is such a potentially great tool that we thought we’d get
into how to tune your engine using this tool not just for max power,
but also to raise the bar for better part-throttle response and
highway cruising. While the innovate meter is capable of logging
up to 44 minutes of brain-numnbing data, we’ll approach this tuning
session assuming you’re going to read the Innovate meter in real
time. Ideally, data logging is better because you can study the
information more closely.
The best place for wide-open throttle
(WOT) air/fuel testing is at the dragstrip. Short of that, you
can do Second-gear bursts of 2 to 3 seconds each and have a passenger
watch the meter. For your initial work, you should shoot for WOT
air/fuel ratios between 2.5:1 to 13.0:1. Remember to make only
one change at a time and keep using the same test procedure. If
you’re at the dragstrip, use mph numbers to help with tuning trends.
As long as your changes improve trap speed, continue to move in
that direction. IN many cases, the combination will be rich – like
11.8:1 at WOT. That means you should start by leaning out the secondary
side of the carburetor. With Holleys and Demons, minimum jet changes
of two sizes per step are a good idea. For example, if you want
to run leaner , go from 80 rear jets to 78s. It’s also a good idea
to start your tuning with the carb in its stock jetting configuration.
IF you are five or six jet sizes (or more) away from the box-stock
configuration, it’s possible there’s something else wrong with
the carb or your engine.
Contrary to what
you may think, street engines spend a majority of time at idel
and at very low throttle openings. In addition, the idle cicuit
continues to deliver fuel even when the carb is delivering fuel
through the main metering circuit. Given this, the best place to
start improving highway and in-town fuel mileage is with the idle
Most general-purpose aftermarket performance carburetors
are designed to deliver around a 12.5:1 air/fuel ratio to avoid
lean surge conditions. Most mild street engines can tolerate part-throttle
air/fuel ratios of 13.5:1 all the way up to as high as 15.0:1.
Keep in mind that all production EFI engines operate 14.7:1 air/fuel
ratio and the driveability is excellent. It’s more of a challenge
to tune a carburetor to achieve a lean 14.7:1 air/fuel at part-throttle
and still deliver excellent and immediate WOT power, but it can
be done. What this means is the power-valve and accelerator-pump
circuits become much more critical. This is where a Q-jet shines,
using its small, highly responsive primary side to achieve excellent
throttle response for part-throttle driving. But other carbs can
be tuned to also work very well. Don’t be afraid of 14.0:1 or even
14.5 – 15.0:1 air/fuel ratios, they don’t hurt the engine. There
is very little load at highway cruise speeds because the engine
is only making about 15 to 25 hp under these conditions.
definitely cut-and-try type work. IF you go too lean on the idle-feed
restrictors, the engine will surge at cruise and hesitate under
light acceleration, since most engines don’t like to accelerate
at lean air/fuel ratios. If you tune the idle-feed restrictors
too lean, the engine will most likely suffer from an off-idle lean
stumble as an early indicator that you’ve gone too far. It’s the
transition circuits that are more seriously challenged and the
ones that will falter first when you begin to lean out the idle
and primary main circuits.
TUNING CHALLENGE 1
Our first real-world
example started out with a 9.0:1 compression 455ci Olds with a
mild hydraulic cam, dual-plane Edelbrock, the stock 455 Q-jet,
headers, and an HEI ignition. The Innovate meter reported an idle
mixture of around 12.0:1 that went lean at about 13.8:1 as the
throttle opened up in mild acceleration. At steady-state highway
cruise speeds, the air/fuel ratio went back rich at around 12.5:1.
At WOT, the engine was actually a bit lean.
The first thing we
did was to adjust WOT tuning by swapping the stock CH secondary
rods to thinner rods (eventually a pair of aluminum rods) until
we had a WOT air/fuel that came in around 12.8:1. Because of the
cam, intake, and header swap, part-throttle transition fuel was
also slightly lean, so we changed to a slightly weaker power- valve
spring to pull the metering rods out of the primary jets sooner
as load increased. The engine responded with a slightly richer
mixture in light acceleration of around 13.2:1, but we still had
a slightly rich cruise air/fuel ratio. When we tried larger (leaner
52B) primary metering rods, it hurt light-throttle acceleration.
Some late-model Q-jets are set up to allow you to adjust the position
or depth of the primary metering rods in the jets. If our carb
had been equipped with this feature, we could have adjusted the
metering rods deeper in the primary jets to lean out the part-throttle
TUNING CHALLENGE 2
We also tried a 350 Chevy equipped
with a long-duration cam and a Holley 750-cfm 0-3310 vacuum-secondary
carb that suffered from a bad off-idle stumble and a pig-rich 10.5:1
ratio at part-throttle cruise. The WOT air/fuel ratio was only
slightly rich at 12.2:1. The challenge was to improve the driveability
and mileage without sacrificing WOT power. After properly adjusting
the primary accelerator-pump linkage, the stumble disappeared.
We also replaced the large 0.036-inch accelerator-pump nozzle to
a 0.028 to further improve the throttle response.
The LM-1 meter
told us the engine was way rich at just off idle, so we first tried
leaning out the idle-mixture screws, but it didn’t help. Next we
disassembled the carb and found that a previous hacker had drilled
out the idle-feed restriction of around 0.032 to 0.035 inch. Some
quick math revealed that the 0.052-inch orifice increased the area
by more than 100 percent. As a temporary fix, we tried a 0.020-inch-diameter
wire stuffed into the idle-fuel jet to reduce the flow area. The
larger wire reduced the area by roughly 15 percent, which leaned
out the part-throttle air/fuel ratio to roughly 11.8:1. which was
still very rich. Further experimenting with reducing the main jets
from 72 to 69 and using a slightly larger 0.025-inch wire in the
idle-feed jet finally got the part-throttle air/fuel ratio close
to 12.8:1. We could not tune any leaner without creating an off-idle
stumble and part-throttle lean-surge condition because of the cam.
The WOT air/fuel also improved the leaner primary jetting to 12.5:1.
These simple changes improved the fuel mileage by over 30 percent.
An ideal accelerator-pump shot is just enough to maximize acceleration.
Additional fuel only kills power.
The best approach is to weld
the supplied bung into the exhaust pipe near or in the header collector.
Mount the Bosch sensor in the upper half of the pipe to prevent
condensation from affecting sensor accuracy.
Part of the Aux Box
software allows you to custom configure up to five gauges on your
laptop such as air/fuel, an rpm signal, and a total of four other
0-5-volt analog channels for stuff like voltage, engine temperature,
engine vacuum (MAP), and throttle position. This data can be viewed
in detail to help you dial in the car.
WOT tuning is relatively
simple. If the meter reads richer than 12.5:1, remove the secondary
float bowl on your Demon or Holley and try jets two steps leader.
Shoot for between 12.5 and 12.9:1 air/fuel or for top dragstrip
Rochester Q-jet WOT is the easiest to tune. All you have to
do is change the metering rods. Fatter rods are leaner (less fuel)while
thinner metering rods flow more fuel for a richer mixture.
The Edelbrock carb primary metering rods and springs are incredibly
easy to change. Merely loosen the lid screw, slide the cover over
and pull the rod and spring out. If you need to change primary
or secondary jets, the lid must come off, which only takes a couple
of minutes and you won’t spill any fuel.
The Q-jet uses one power-valve
piston with a hanger to operate both metering rods in their respective
primary jets. A stiffer power-valve spring pushes the metering
rods out of the jets sooner (at higher vacuum levels). A softer
power-valve spring uses less fuel by keeping the metering rods
buried in the primary jets.
Setting idle mixture will change
when you bolt on the air cleaner. If possible, set your idle-mixture
ratios with the air cleaner in place. Holley and Demon carbs use
a diaphragm-style primary power valve. The opening point is usually
marked on the valve, such as a number like 65. This means that
the valve opens (adding more fuel) at 6.5 inches of manifold vacuum.
Using a lower-number power valve, like a 55 or 45 delays power-valve
opening and could improve fuel mileage.
While the idle circuit
is very basic, it has a huge impact on drieability. The idle-
mixture screw controls the idle air/fuel mixture, but once the throttle
is open the idle fuel restrictor combined with an idel air
determine the off-idle air/fuel ratio until the throttle blades
open enough to create fuel flow out of the boosters and the
main metering circuit.
Big cams with low idle manifold vacuum require
opening to flow sufficient air to make the engine idle. If
the throttle blades expose too much of the idle transfer slot (arrow)
at curb idle. Then the usual fix is to drill two small holes
(starting at around 3/32 inch) in the primary throttle blades on the idle-port
slide of the blades. This shows the proper amount of the idle
CC Tuning Tip
Poor ignition performance
can be confusing when using the LM-1 air/fuel ratio meter. When
a cylinder misfires, the air and fuel do not combust, meaning the
oxygen in the air moves into the exhaust where the O2 sensor will
immediately pick up the additional air and indicate a lean condition.
Simply put, this means a dead cylinder or occasional misfire will
show up on the innovate meter as a lean condition not a rich mixture
as you might thing. Fix the misfiring cylinder(s) and the air/
fuel ration will read as a richer mixture.
Innovate Motorsports; Irvine, CA; 949/502-8400;