Tune a Holley
by Lars Grimsrud
Musclecar, Collector & Exotic Auto Repair &
Rev. New 3-14-01
This tech paper will discuss basic set-up and
tuning of Holley vacuum secondary carbs for optimum street performance and
Holley carbs have been used by hot
rodders for many decades now. Holley offers a very tuneable, modular carb
design, yet it’s surprising how few people understand the basic
tuning principles and procedures for these carbs. This tech paper will
discuss some basic tuning procedures and techniques to help you get your
setup optimized a little better.
Holley carbs have several
operating systems, all of which can be tuned independently. Often, an
improper setup of one system will give false indications of a problem in
another system. Attempting to cure Holley problems by tuning the wrong
system results in a really messed up carb, and a lot of frustration for
tuners and car owners.
Holleys have the following basic operating
These systems at times
overlap in their operation. Not only does each system need to be properly
tuned, but its timing and “overlap” with other systems is
critical to proper performance.
Tech Tip #1
Before you go
trying to fix all the errors of the previous carb tuner, set your carb up
to the stock Holley spec for your carb List Number (stamped into the front
of the air horn). Holley does a pretty decent job of supplying a
ready-to-run carb, so the jetting, pump cam, shooter sizing, and spring
rates are usually pretty close. This will give you a good baseline
starting point for your tuning process. You can either call your local
Holley distributor for the stock specs on your List Number carb, or go to
this website to check out the baseline
When buying tuning parts for your Holley you have two
Obviously, Holley offers original parts at the Holley price.
I highly recommend original Holley rebuild kits, gaskets, and power
valves. BG, or Barry Grant, also makes parts for Holleys, and offers them
at a fraction of the Holley price. BG is a great source for items such as
the accelerator pump cam kits.
Tech Tip #3
musclecars (with a only a few notable exceptions) came from the factory
with a dual plane intake manifold, also known as a
“180-degree” manifold. A dual plane intake is characterized by
having two separate sides (right and left) which feed two different
“levels” inside the manifold. Half of the cylinders are
“fed” from the one level, and the other half are
“fed” from the second level. A dual plane manifold promotes
higher intake velocity at lower rpm, improving low-end torque and low-end
throttle response. A dual plane manifold also promotes very good vacuum
at idle and at low speed.
At very high rpm, a dual plane manifold
will loose some efficiency. For this reason, there are single plane
manifolds available. Known also as “360-degree” manifolds,
the single plane manifolds typically have large intake runners and a
large, open plenum under the carb. Very good for high-rpm use, these
manifolds can be a real challenge to tune for low-end, street rpm use. A
single plane manifold will typically significantly reduce vacuum, and this
causes several problems.
Intake vacuum is used by the carb as a
“signal” for many different events to occur. As vacuum is
reduced, caused by large plenum intakes and large overlap cams, the vacuum
“range” becomes much more narrow. The carb tuner must make
the various events occur within this narrow tuning range, while hitting a
much more narrow “tolerance” for overlapping events to occur.
Typically, a large-plenum intake will require richer main jetting and a
richer pump shot to avoid a lean stumble hesitation upon initial
acceleration. The idle transition may even require extensive recalibration
of the idle restrictors – not an operation for the
My recommendation: don’t use a single
plane intake on a street driven car unless you have a lot of cubic inches
and you’re willing to put in a lot of time tuning the idle
transition. If you’re experiencing an off-idle stumble on a car
with a single plane manifold, your problem may be in the manifold and not
with the carb. Consider spending the $$ for a dual plane intake and saving
yourself some frustration.
Tech Tip #4
Which way to tune the
accelerator pump – richer or leaner?
Hesitation, stumble, or
backfire is usually indicative of a lean condition.
throttle” is usually indicative of a rich condition.
condition may also be accompanied by a small puff of black smoke upon
Tech Tip #5
When removing the float bowl,
pull one of the lower float bolt screws first. Place a small container,
such as the plastic cap off of a spray paint can, under the float bowl,
and catch the fuel as it drains out. When you put it all back together,
you can use a small kitchen funnel (just don’t tell your wife about
it) to pour the drained fuel back into the bowl by dumping the fuel down
the bowl vent tube (the tube sticking up right at the choke).
When using a Holley on a car with a bumpy cam, it is often
necessary to screw the idle speed screw in far enough that the ported
vacuum slot in the throttle bore is uncovered, producing significant
ported vacuum to the distributor vacuum advance at idle. When the engine
is rev’ed up, the vacuum source advances the timing, as it should.
When the throttle is closed, the rpm will very gradually bleed down as
vacuum advance gradually fades: the engine will not settle down
immediately to the correct idle speed. To eliminate this problem, you can
adjust the secondary opening screw to allow the secondary throttle plates
to be cracked open a little more than the spec requirement. This will
bleed a little air through the secondaries at idle, increasing idle speed
and allowing the primaries to be closed down to correct the ported vacuum
Tools and Equipment Required
As a minimum, you will
need the following tools:
1. Vacuum Gauge
2. Small cup to
drain fuel into
4. Box end wrenches
plug removal tools
Here is my
recommended sequence and procedure for doing a basic Holley
1. Set the float level.
You’ll be amazed how many
people try tuning a Holley without ever checking the float level. An
incorrect float level can give you all kinds of symptoms and problems, so
get this one set right off the bat. Most Holleys have a sight plug on the
side of the float bowls, and have externally adjustable floats. With the
engine HOT and running (float level will change with engine temperature
- make sure you’re completely warmed up before setting this),
put a rag under the sight plug area and pull the plug. Adjust the float
level until fuel just barely starts to dribble out of the plug, then lower
it until the dribble barely stops. The slotted screw in the center of the
adjustment is the locking feature – loosen it and turn the hex nut
CW to lower the float level, CCW to raise it.
2. Make sure your
Power Valve is in the ballpark.
Most “stock” Holleys come
with a power valve in the 6.5″ range. This means that the power
valve will open and start dumping fuel into the discharge when manifold
vacuum hits 6.5″ Hg (Mercury) of vacuum. A lot of cars with big
overlap cams, single plane manifolds, or operating at high altitude will
not pull 6.5″ vacuum with the transmission in “drive.”
This means that the power valve is flooding the engine with fuel at idle,
and you’ll never get the tuning right unless you fix it.
up a vacuum gauge to the intake manifold or to one of the vacuum ports at
the base of the carb. With the transmission in “drive” (or in
neutral on a manual car), note the vacuum reading. Your power valve
should be rated at about 2″ below the actual vacuum reading. On a
car running at 6″ of vacuum in drive, you will need a 4.5 or a 3.5
3. Now get the main metering circuit in the
Now that your power valve is not flooding you out, you can
get the main metering jets tuned in. If your main metering circuit is
either too lean or too rich, you will not be able to properly set up the
accelerator pump circuit. Trying to fix a bog on acceleration with pump
cams and discharge nozzles when the main metering is off is like chasing
your tail. The main metering system also affect the idle circuit, so you
need to get the main jets in line before going on to the other tuning
Pop a new set of spark plugs in the engine (don’t
use the new Platinum or other exotic metal plugs- they won’t read
right). Now take the car out for a drive around a few blocks. Get it up
to steady-state cruise rpm, and make a few moderate accelerations. Pull
the car back into the garage, allowing it to idle as little as possible.
Shut it down and pull a few of the plugs. The plugs should have a very
light, off-white or tan color around the nose of the insulator. If
you’re pure white (completely clean), you’re probably running
too lean. If you’re picking up black soot, you’re too rich.
Raise or lower primary jet sizes by 2 sizes at a time and repeat this test
until your plugs are showing the correct color.
4. Tweak your idle
Screw both of the screws in until they gently bottom out, and
turn them both out 1-1/4 turns. This is a good starting point. With a
tach on the engine to monitor rpm, start the engine and turn the screws �
turn at a time, turning both screws the same amount, until the best rpm
reading is observed. Then turn them in 1/8 to � turn to lean it out just
a tad. Once set, I like to shut the engine off and turn both of the
screws all the way in, counting the turns, until they gently bottom out
again. Just to make sure they’re both set the same. Then back them
out to the setting you had.
5. You’re finally ready to
fine-tune the accelerator pump circuit.
Many people start by tuning
this, and can never get the stumble tuned out of the car. By making sure
that the other systems are approximately right, you can now set this
system up so that it’s not trying to compensate for other
There are two tuning parameters to the accelerator
pump circuit: Pump Cam and Discharge Nozzle.
The pump cam, a
little plastic, color-coded part located on the throttle shaft with a
single screw, will determine the total volume of the pump shot.
The discharge nozzle, or “shooter,” is available in
different sizes, and determines the rate of discharge. In other words, it
is possible to install a large-volume pump cam, and have a small-diameter
“shooter” which “bleeds” the total volume in over
a longer duration. Or you can install a big “shooter” which
allows the entire pump shot to be dumped in almost immediately.
There is no real scientific way of coming up with the exact
correct combination for these parts. As a rule, of the car stumbles,
backfires, or hesitates on initial throttle opening, try a larger shooter
(the size is stamped on the body, and they’re available in
.001″ increments). If the car has a “lazy throttle,”
indicative of a rich condition, try a smaller pump cam at first.
The last step in the initial tuning process is to set the secondary
The secondary opening rate is determined by the spring
in the secondary diaphragm. A soft spring will allow the secondaries to
open sooner. Install softer springs until the car develops a slight
stumble from the opening rate, then go back one spring level. This will
allow the secondaries to open as quick as possible while avoiding a
A common mistake made by “performance” tuners
is to remove the secondary diaphragm check ball. The check ball is
installed in the secondary diaphragm housing, and helps to control the
secondary opening rate. Removal of the ball will result in immediate
opening of the secondaries with very little mass air flow through the
primaries, with a resultant massive stumble upon acceleration.
cannot normally get the vacuum secondaries to open by flicking the
throttle and rev’ing the rpm with the car in neutral. This is not a
valid test of the functionality of the secondaries.
7. Now, if you
want to fine-tune the primary jetting, you can do one of three
For maximum horsepower, you will need to run the car through a
measured distance and note its speed at the end of the distance. Using one
of the available performance computers, or running the car at a track, is
best for this. The correct size jets are the jets which give the car the
highest mph at the end of the measured distance (not the shortest elapsed
time). Again, change jet sizes 2 sizes at a time to see if the speed
increases or decreases.
For maximum economy, drop the jet size
until the car develops a slight stumble upon acceleration, then go up two
For a really good, scientific way of setting up the jetting,
check out this
& Technical Assistance
If you have questions or comments regarding this
article, or if you notice any errors that need to be corrected (which is
quite possible since I’m writing this from memory…), please
feel free to drop me an e-mail. Also, if you need any technical
assistance or advice regarding this process, or other maintenance issues,
feel free to contact me: