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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: