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adjust-a-jet metering block kit contents
accelerator pump for a carburetor with extended arm
removing jets from a carburetor metering block
side of a holley carburetor with fuel bowl removed
adjust-a-jet metering block on carburetor
adjust-a-jet metering block installed on a holley carburetor

This is all you need to give your Holley an externally adjustable fuel metering system. Each Percy's Adjust-a-Jet kit converts one metering block or plate.

After removing the fuel bowl, you will need to replace the stock accelerator pump arm with the kit's extended pump arm. Use a 1/8 inch punch and hammer to tap the arm roll pin out just enough to remove the old arm, then install the new arm and tap the roll pin back in.

Remove the jets from the carburetor's metering block.

Scrape any residual gasket material off the block, then place one of the kit's new gaskets on the block surface facing the fuel bowl. Percy's also includes a foam insert, which goes between the metering block and plate. This insert is designed to prevent fuel slosh.

Place the Adjust-a-Jet on the metering block as shown.

Slide the kit's nylon fuel bowl gasket on the bolts. Hold the other gasket on the new plate, then attach the fuel bowl and tighten the bolts. You may have to adjust the accelerator pump arm to make sure it is in correct position. Insert the kit's O-ring in the metering needle hole. Carefully turn the needle with your fingertips to the fully closed position. Tighten the needle's hold-down nut until it applies a slight amount of pressure on the O-ring. Now you're ready to make quick, accurate air/fuel adjustments.

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[Editor’s note: This article was originally posted in 2012. Since then, the Percy’s company was purchased by Taylor Cable. Taylor is still making Adjust-a-Jet kits, but now under the Taylor name, with many of the same part numbers. You can find them by clicking here.]

Remove fuel bowl, swap out jets, replace fuel bowl, start car, and test. Remove fuel bowl again, swap out jets again, replace fuel bowl, start car, and test again. Remove fuel bowl yet again, swap out—well, you get the idea. Making fuel adjustments to your carburetor can be tedious and frustrating, not to mention messy and time-consuming.

Fortunately, Percy’s High Performance has taken much of the hassle, mess, and guesswork out of tuning most Holley carburetors. Whether you own a hardcore race car or a “double duty” street/strip car, the Percy’s Adjust-a-jet Metering Plate allows you to make adjustments to your carburetor’s air/fuel mixture instantly without removing the float bowls. In fact, you race guys can make quick, accurate, in-between round adjustments at the track while the engine is still running!

Simple Installation

The Adjust-a-Jet set comes with everything you need to make the installation quick and painless, including the metering plate, bolts, accelerator pump arm, gaskets, and O-rings. It bolts on between the existing metering block and fuel bowl with little or no modification required.

To get started, you’ll have to remove the fuel bowl–for the last time. Once the fuel bowl is off, you replace the existing accelerator pump arm with the special extended accelerator arm included in the kit. You then remove the existing jets, put the Adjust-a-Jet and new gaskets in place, and reinstall the fuel bowl. It’s all stuff you can do with basic hand tools, and it’s all covered in the step-by-step slide show above.

Easy Adjustments

Once the Adjust-a-Jet is installed and you have established your initial setting, you can make tuning adjustments with simple fractional turns of the external needle valve.

To establish your initial setting, turn the Adjust-a-Jet needle to the fully closed position. We recommend that you turn the needle adjustment screw with your fingertips to prevent overtightening. Once the needle is in the closed position, add the needle O-ring and secure it with the included hold-down nut.

Using the embossed markings around the needle assembly, begin turning the Adjust-a-Jet needle in 1/8-inch increments until you’ve reached the desired setting. The chart below compares Holley jet sizes to fractional turns of the Adjust-a-Jet valve:

Jet Size=Turns Open

#60 (jet size) =1 (turns open)
62=1 1/8
64=1 1/4
65=1 1/2
66=1 5/8
67=1 3/4
68=1 7/8
69=2
70=2 1/8
71=2 1/4
72=2 3/8
73=2 1/2
74=2 5/8
75=2 3/4
76=3
77=3 1/2
78=3 5/8
79=3 3/4
80=4
81=4 1/2
82=4 3/4
83=5
84=5 1/4
85=5 1/2
86=5 5/8
87=5 3/4
88=6
89=6 1/2
90=7
91=7 1/2
92=8
93=8 1/2
94=9
95=9 1/2
96=10 1/2
97=11 1/2
98=12
99=12 3/4
100=13

Once your initial setting is dialed in, you can adjust for rich/lean conditions as opposed to jet size. Best of all, you can make these adjustments with the carburetor in place and the engine running, so your tuning will be as accurate as possible.

Here’s a listing of Percy’s popular Adjust-a-Jet applications:

PHP-15001 Percy’s Adjust-a-Jet, Holley 2300/3310/4150/4160/4150HP, street/strip gasoline
PHP-15002 Percy’s Adjust-a-Jet, Holley Dominator, street/strip gasoline
PHP-15003 Percy’s Adjust-a-Jet, Holley 4150 and Dominator, drag racing alcohol
PHP-15004 Percy’s Adjust-a-Jet, Holley 4150/4160 with circle track primary
PHP-15005 Percy’s Adjust-a-Jet, Holley 4150/4160 with circle track secondary
PHP-15006 Percy’s Adjust-a-Jet, Holley 4150/4160 with circle track primary, alcohol
PHP-15007 Percy’s Adjust-a-Jet, Holley 4150/4160 with circle track secondary, alcohol
Keywords
Percy's Adust-a-Jet
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Author: David Fuller

David Fuller is OnAllCylinders' managing editor. During his 20-year career in the auto industry, he has covered a variety of races, shows, and industry events and has authored articles for multiple magazines. He has also partnered with mainstream and trade publications on a wide range of editorial projects. In 2012, he helped establish OnAllCylinders, where he enjoys covering all facets of hot rodding and racing.