Fuel Bowl Control

December 19th, 2011

By Liz Miles

Here are the lock screw and adjustment nut with instructions.

The process of carburetion starts at the bowl where fuel arrives at the needle and seat. The needle’s cone-shaped top presses against the matching seat in the housing to limit the amount of fuel that enters. The float located inside of the bowl rides on the fuel like a boat and raises or lowers the needle based on the level. This is why proper float adjustment is crucial. If the float is adjusted to ride too high, the excess fuel will find its way to the vent tube and into your engine unatomized, causing the engine to run poorly or stall. If the float level is set too low, you can starve your engine of fuel, causing it to lean out or stumble. Once the fuel level is set properly through the float adjustment, it’s up to the bowl’s interior design to help keep the fuel under control. Holley’s new all-aluminum Ultra HP houses several upgrades to assist with this control. Baffling inside of the bowl and a shelf under the needle and seat have been added to help stabilize fuel under typical racing conditions, such as launching, turning and heavy braking. The bowl’s floor has been recontoured to direct fuel to the main jets, further refining the fuel’s flow and keeping the jets covered. These new bowls have a 20-percent higher capacity than the traditional V-bowl design for more fuel on demand. Even the fuel level sights have been improved with glass Pyrex windows on both sides to ensure safe and easy fuel level adjustments.

With all of these new features, Holley’s carburetors make taking control of fuel easier and more efficient than ever. Your only job is to properly set the float level. There are two methods of making float adjustments for Holley performance carburetors, depending on the style of the float bowl and needle and seat used. They are the internal (dry) setting and the external (wet) setting. Adjusting the internal float is accomplished via the fuel bowl off of the carburetor. With internally adjustable needle and seats, the fuel bowl is flipped upside down, and the float tang, or tab, is adjusted to the point where the float surface is parallel to the bowl surface just below. An initial dry setting can also be accomplished with externally adjustable needle and seats, which are most typical of current performance carburetors. This also starts with flipping the bowl upside down and adjusting the nut on the top (now flipped to the bottom) of the bowl until the float is level with the bowl body.

Holley’s new bowl in the new all-aluminum Ultra HP carbs feature built-in baffling and room for 20-percent more fuel than traditional V-bowl designs.

A wet level float adjustment can be performed on float bowls hung from either the center or the side when the carburetor is equipped with external needle and seat adjustments. First, start the vehicle (in a well-ventilated area) and allow the idle to stabilize. At that point, shut off the engine and remove the sight plugs if so equipped. The fuel level should be just under the lowest point of the plug, so if fuel comes out when the plug is removed, it’s currently set too high. If you can’t see the fuel level, grab a flashlight and rock the vehicle to see movement. If your carburetor has clear sight windows, the fuel level should be in the middle of the glass.

To adjust the level, replace the sight plugs, stabilize the adjustment nut with a 5/8-inch wrench and crack the locking screw loose with a large, flat-blade screwdriver at the top front of the bowl. Use the wrench to turn the adjusting nut clockwise to lower the float level or counter-clockwise to raise it. Turn the nut in small increments—1/8th turn to start—as turning changes float level with very little adjustment. Relock the screw while holding the adjustment nut still. Run the engine again to allow the new float level to stabilize and recheck. Keep doing this until the fuel level is in the desired location.

Just a reminder: The float adjustment feature on Holley carburetors cannot cure an engine that is running poorly because of a bad ignition system, a clogged fuel filter, an improperly operating fuel pump or a fuel pressure regulator that is set too high or too low. The adjustment is provided solely to ensure that the fuel in the bowl is at the ideal level for the carburetor to work correctly.

If the fuel level is within specifications and you’re still having problems, over-filling the fuel bowls of your fuel pressure regulator may be to blame. It can force excess fuel into the carburetor while the needle is off its seat. This wreaks all sorts of havoc. Holley carburetors come preset from the factory, but it’s up to you to do any fine-tuning for your application.

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