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Adjusting Your Carburetor

By: Holley06/04/2019 < Back to Blog Home
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GENERAL GUIDELINES FOR ADJUSTING BRASS AND NITROPHYL FLOATS


Two methods of float adjustment are provided for with Holley performance carburetors depending on the style of float bowl and needle and seat assembly employed. They are the internal (dry) setting and the external (wet) setting. The internal float adjustment is accomplished with the fuel bowl off the carburetor. With “internally adjustable” needle and seats, the fuel bowl is inverted and the float tang, or tab, is adjusted to the point where the float surface is parallel to the fuel bowl surface, just underneath. An initial dry setting can also be accomplished with “externally adjustable” needle and seats. To achieve this, invert the fuel bowl and turn the adjusting nut until the float surface lies parallel to the fuel bowl casting surface underneath.


Another, more accurate adjustment can be made with the side hung style float if measuring gauges, such as drill bits, are available. Here, with the fuel bowl inverted, the primary float can be adjusted to the point where there is a 7/64" gap between the “toe” of the float and the bottom of the fuel bowl surface underneath. The float “toe” is the part of the float furthest from where the arm is attached. The secondary float can be adjusted to the point where there is a 13/64" gap between the “heel” of the float and the bottom of the fuel bowl surface underneath. The float “heel” is the part of the float closest to the point where the arm is attached.


A “wet” level float adjustment can be performed on either the side or center hung floats, if the fuel bowls have provision for the externally adjustable needle and seats. This adjustment is made as follows. Start the vehicle up and move it out of the garage and into an open area where plenty of fresh ventilation is available. Allow the idle to stabilize. Turn the engine off and remove the sight plug from the primary fuel bowl to inspect the fuel level. If it’s been determined that adjustment is required use a large screw driver to crack loose the lock screw. With a 5/8" open-end wrench turn the adjusting nut clockwise to lower the float level.


Conversely, turn the adjusting nut counter-clockwise to raise the float level. Tighten the lock screw. Restart the vehicle and let the engine idle stabilize. Shut the engine off. Remove the sight plug to reinspect the fuel level. The fuel level should stabilize at just below the level of the fuel bowl sight plug hole. This same adjustment procedure is performed on the secondary bowl.


GENERAL GUIDELINES FOR ADJUSTING DURACON (PLASTIC) FLOATS


The Duracon float rides higher on the fuel than either the brass or nitrophyl float and, therefore, a higher setting is in order. A Duracon float, set at the same level as either a brass or nitrophyl float, would make the carburetor run leaner, everything else being equal. This is because there would be less fuel available in the fuel bowl. The Duracon float setting must be higher to compensate for this condition.


Dry Setting for Duracon Center Hung Float: The primary side setting is .3125" (5/16"), measured with the fuel bowl inverted, at the middle of the float. The secondary side setting is .3750" (3/8"), measured with the fuel bowl inverted, at the middle of the float (back side).


Dry Setting for Duracon Side Hung Float: The primary side setting is .2188" (7/32"), measured with the fuel bowl inverted, at the toe of the float. The secondary side setting is .3125" (5/16"), measured with the fuel bowl inverted, at the toe of the float.


Wet Setting for Duracon Float: Refer to “Wet Float Setting”, discussed previously


WARNING: Caution should be exercised when doing the wet level float adjustment. Fuel at the needle and seat is under pressure from the fuel pump. Some may leak out when the adjustment is made and shop rags should be available to immediately wipe up any fuel spillage. Gasoline is flammable and proper precaution should be taken.


CAUTION: Once again, remember that these are general guidelines for adjusting floats. Your particular application may require additional fine tuning over and above these listed procedures


NOTE: The float adjustment feature on Holley carburetors cannot cure a poor running engine, a bad ignition system, a clogged fuel filter, an improperly operating fuel pump or fuel pressure that is too high or low. This adjustment is provided solely to ensure that the fuel in the bowl can be adjusted to the correct level for the carburetor to perform its function. There is no need to “wrench” excessively on the adjustment nut. A quarter of a turn one way or the other should be enough to bring you into spec.