How to Lean a Holley Carb

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A Holley carburetor can be leaned either through the idle circuit or the main jets. The carb has main and secondary replaceable jets with its sizing numbers on its face. Leaning out a carburetor will increase the fuel economy and possibly increase the power to a certain point. In order to check for the proper jetting at the main jets, new spark plugs should be installed. The vehicle should not be idled for a lengthy period, and should be driven hard through first and second gear and shut down as soon as possible. The plug should be removed and inspected for color. To lean a mixture will cause destructive detonation, which must be avoided.

Things You'll Need

  • Common screwdriver
  • ¼-inch drive ratchet
  • Set of ¼-inch drive sockets
  • 3/8-inch drive spark plug socket
  • 3/8-inch drive ratchet
  • Vacuum gauge
  • Box of assorted jets
  • Remove the air cleaner element. Grab the throttle linkage and open the throttle slightly and release. This will close the choke and set the fast idle circuit. Remove the lower, front transmission vacuum port and connect the vacuum gauge to this port (the hose just pulls off).

  • Start the engine and allow it to warm up to normal operating range. Observe the choke operation. The choke should be fully closed to start with. As soon as the engine starts, the vacuum pull-off cracks the choke open a small amount--approximately 15 degrees. As the engine warms up, the choke opens progressively to the full open position. A quick throttle movement allows the fast idle cam on the passenger side to move down, allowing a normal idle. Replace the vacuum pull-off if inoperative.

  • Adjust the choke by loosening the screws and rotating the choke spring housing when the engine is dead cold. If the choke does not close all the way, hold the throttle open slightly and rotate the choke spring housing clockwise just enough to close the choke. If the choke does not open all the way, turn it counterclockwise until the choke begins to open.

  • Monitor the vacuum gauge with the engine at idle. Turn the driver's side front idle mixture adjustment screw in until the vacuum starts to drop, then turn it out until the highest vacuum is achieved. Do the other side in the same manner. If the carburetor has 4-corner mixture, adjust all mixture screws the same way. Once done, repeat the procedure one more time. Now the mixture is perfect in the idle circuit.

  • Check the mid-range circuit. The mid-range circuit is where the fuel economy and power lies. If an oxygen sensor is not installed, which is not likely in a carbureted car, the condition of the plugs will show the mixture. Run the vehicle to the redline through first and second gear. Try to shut it down as soon as possible. Pull the plugs and check the plug color. A black color forming will be too rich. A blistering, white or tan color is too lean.

  • Lean out the main jets (this is the more lengthy, but equally effective method, and is optional) by replacing them with jets that are two numbers smaller. Run the vehicle, listen for detonation, and feel for power increase or decrease. Keep replacing the jets until detonation starts, and back off the jets to the next smaller size.

  • Replace the jets by removing the float bowl. Notice that there are four bolts in the front corners of the bowl. Drain the fuel out first by placing a rag under the bowl and removing one of the lower bolts so the fuel will drain out. Remove the rest of the bolts and pull the bowl off. The jets are obvious in the lower part of the metering block. Use a common screwdriver to remove them. Do not over-tighten the jets or mess them up by slipping with the screwdriver and scratching them.

  • Photo Credit Jupiterimages/Photos.com/Getty Images
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