How to Adjust Tune Holley Carb Carburetor Tutorial Instructions

How to Adjust Tune Holley Carb Carburetor Tutorial Instructions


Brought to you by Jegs. Today’s carburetors
from Holley offer better out-of-the-box performance than ever with models available that are suitable
for a wide range of engine types, sizes, and applications. Once you’ve selected the right
Holley carb for your engine, some basic tuning steps will be necessary to ensure your carburetor
performs at its best. Before you begin, you’ll need to gather a
few tools. Unpack the carburetor and examine it to get familiar with it. You may notice
some fluid remaining in the carburetor or a solvent smell, this is normal. Every Holley
carburetor is factory flow tested before shipment and a small amount of liquid will remain.
Please note that Holley mechanical-secondary “Double Pumper” carburetors are not recommended
for use with automatic overdrive transmissions or Ford automatic transmissions that require
a kick-down lever. To tune your carb, you’ll need a vacuum gauge, a tachometer, a Philips
screwdriver, an assortment of wrenches, a Holley Jet tool, and Holley’s small parts
and tuning kit for your model carburetor. Before you begin, you should also make sure
your ignition components like spark plugs, plug wires, distributor cap, and rotor are
all in good condition and that the ignition timing is properly set. Your Holley carburetor is designed for a fuel
pressure range of between a high of seven PSI at idle and a low of five PSI at wide
open throttle. So check your fuel pressure and adjust the regulator if applicable. Inspect
your air and fuel filters for cleanliness and ensure that there are no vacuum or fuel
leaks in any hoses or gaskets. Take a few moments to familiarize yourself with the instructions
provided with your new Holley carburetor. It will help you identify the different components
of your carb and the various vacuum and fuel fittings. Follow along as we show you how
to adjust fuel level, set idle mixture, install the correct power valve, adjust the accelerator
pump operation, and set basic jetting. The first step is setting the proper fuel
level in the float bowls with the vehicle on level ground with the engine running, remove
the primary float bowl site plug losing the fuel lock screw and turn the adjusting knot
clockwise to lower the fuel level and counterclockwise to raise it. When fuel slowly trickles out
of the site hole, the level is properly set. Tighten the lock screw and replace the site
plug then repeat the process on the secondary float bowl. Use caution to prevent excessive
fuel spillage and potential fires. With the fuel level set, adjust the carb idle speed
screw until the engine is idling at the desired RPM. Before installing your carburetor, you
should make sure both idle mixture screws are set to full lean turned all the way in
clockwise then backed out one and a half turns. With your vacuum gauges attached to the manifold
vacuum port, adjust both idle mixture screws one-eighth of a turn at a time alternating
between each screw until the highest vacuum reading is achieved. With the idle mixture
set, adjust the carb idle screw to return to your desired idle RPM if necessary. The
idle vacuum reading will determine the appropriate power valve size for your engine. For automatic
transmissions, measure with the car in gear and your foot on the break. For manual transmissions,
idle with the gearbox in neutral or the clutch disengaged. If the reading is up to 12 inches
of mercury, divide by two and round up to the next available size. If it’s over 12,
use a 6.5 power valve. The power valve and main jets are located on the primary metering
block while the secondary jets are located in the secondary metering block. To access the power valve or jets, you’ll
need to remove the appropriate float bowl. With the engine off and something handy available
to capture spilled fuel, remove the float bowl to access the metering block. The power
valve simply screws into the carburetor side of the metering block while the jets are accessed
from the float bowl inside. Use the proper jet tool rather than a flat bladed screwdriver
prevents damage to the jets which can harm performance. Stock Holley carburetor jetting
is generally a good starting point. For most double pumpers, a spread of six sizes between
primary and secondary jets is recommended, but you can go as high as eight sizes to run
rich best at wide open throttle for added protection against detonation. Tuning is best
performed on an engine or chassy dyno but for most owners, the next best place is at
the track where you can safely and consistently run your car. Most 200 to 300 horsepower engines
will run well with 64 primary and 70 secondary jets as a starting point. You can further
turn your primaries by driving at part throttle on a long stretch of road so that the secondaries
are closed. Jet down your primaries in two-size increments until you feel the engine surge
at part throttle then jet up by two sizes for the lean best setting. Secondaries can
be set at the track by jetting up or down in two-size increments and keeping an eye
on elapsed times and trap speeds. The accelerator pumps are the key to proper
throttle response and drivability. Best performance is achieved by delivering the pump shot at
the exact rate required by the engine. Accelerator pump adjustment begins with making sure that
the slightest movement of the throttle results in movement of the pump operating lever. This
is adjusted by compressing and relaxing the pump override spring. Make sure that the fast
idle cam lever is disengaged so that the operating lever opens all the way. Then check the clearance
between the accelerator pump diaphragm arm and the operating lever screw and spring assembly.
The arms should barely be touching the bottom of the screw with no slack. Insert a 15,000s
filler gauge between the pump arm and screw bottom. The pump arm should move down slightly
with the gauge inserted. Tighten or loosen the adjusting screw as appropriate. The next
step is selecting the right nozzles. The size of the nozzles determines the rate and amount
of fuel that’s delivered. Heavy cars with tall gears will typically use smaller numbered
nozzles that deliver fuel gradually, while vehicles with a higher power to weight ratio
or shorter gearing will prefer larger nozzles that deliver fuel more quickly. Start by increasing
the nozzle size by three numbers. After each nozzle change, run the car under normal road
conditions and quickly open the throttle to see how the carburetor reacts. Continue to
experiment with nozzle changes until the throttle response is crisp and clean. Double pumpered
carbs require secondary nozzles sized to supply fuel for a sufficient length of time so that
the secondary jets can begin to deliver fuel once the secondary throttles are open. Once
nozzle size has been determined, you can further tune the accelerator pump response by changing
pump cams or changing the cam position. Placing the screw in position one activates the pump
earlier which helps engines with a relatively low idle get the right fuel as the throttle
is open, while two and three delay the pump action for engines with the higher idle. There are many ways to fine tune your Holley
carburetor for best performance on the street or at the track. For additional questions
or more information about Holley carburetors, call and speak to a Jegs tech at 1-800-435-4545.
Brought to you by Jegs.

About the Author: Michael Flood

3 Comments

  1. clear plastic sight plugs are plastic and snap-off very easily, then you get to remove the carb to access the broken plug

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