Tuning a 390 CFM Holley is a simple process thanks to Holley's modular design. This means that the 390 CFM (cubic feet per minute air flow rating) is simply sized up to a larger CFM for larger displacement engines. The tuning on a 390 CFM is the same as on a high-performance 1050 CFM Holley carb. The primary tuning process starts with checking the fuel level and then adjusting the air-fuel mixture. You may also need to tune the jets and power valve based on engine performance increases or changes in elevation/air temperature conditions.
Start the engine and place the vehicle on a level surface.
Remove the fuel level sight port from the side of the fuel bowl. Once the screw is removed with a screwdriver, you'll want to watch the hole for a slight flow of fuel coming out of the hole.
Turn the float adjustment screw out two turns. Pull up on the adjustment nut by hand or with pliers until the nut's seal to the fuel bowl breaks. At this point, you may turn the nut counterclockwise with an open-end wrench to raise the fuel level, or clockwise to lower the fuel level until a slight trickle of fuel comes from the sight port hole on the side of the bowl.
Tighten the adjustment screw with a screwdriver while keeping the adjustment nut stationary. Insert the sight port screw and tighten.
Turn the driver-side idle mix screw clockwise until it bottoms out. Complete the same process on the passenger-side idle mix screw. These screws are found on the metering block, which is positioned between the fuel bowl and the main body of the carb.
Turn the screws counterclockwise exactly 1.5 turns to establish a baseline measurement for your upcoming vacuum reading.
Attach a vacuum gauge to the vacuum port on the intake manifold. At this point, you may start the engine and allow it to idle while observing the vacuum gauge. Stock engines will run with a vacuum level around 20. Higher performance engines will operate as low as 7 or 8, due to their longer-duration cams.
Turn one of the idle mix screws a half turn, then move to the opposite screw and complete the same half turn, both in a counterclockwise or loosening direction. You will watch the vacuum gauge after each half-turn of the idle screws for an increase in vacuum pressure. Once the vacuum level ceases to increase, you've reached the proper adjustment of the idle mix.
Remove the four bolts from the fuel bowl with a 5/16-inch socket wrench. Preserve the gaskets if you can, ensuring that if they rip or appear fatigued, that you replace them with a new set.
Remove the power valve with a 1-inch box wrench and the two main jets with a screwdriver. Note the two-digit size rating on the power valve and jets. The power valve number signifies the vacuum level at which it's engaged during acceleration. The jet numbers indicate the diameter of the jet hole. A larger jet number means increased fuel flow.
Insert the power valve that matches your engine's vacuum. You may find this number by dividing the maximum vacuum reading found during the idle adjustment process by 2. For example, if the maximum vacuum reading was 17, dividing by two yields 8.5. Insert the power valve with the numbers 8 and 5 stamped on its head.
Increase the jet size by one number if the ambient temperature dips below 1.67 degrees Celsius, or if the elevation decreases by 2,000 feet from your standard operating zone. Decrease the jet size in the opposite scenario, where the ambient temperature is above 40.6 degrees Celsius, or the elevation is 2,000 feet higher than normal. Screw the jets in with a screwdriver until tight. Tighten the new power valve into the power valve cavity with a box wrench.
Install the fuel bowl on the metering block and tighten the bowl bolts to 8 foot-pounds with a torque wrench.