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Rebuild a Holley Carburetor

Here's A Step-By-Step Rebuild Tour Through A Typical Holley Vacuum-Secondary Four-Barrel Carburetor

Jeff Smith Aug 1, 2001

Step By Step

Let’s start by identifying the major components of a Holley carburetor: (A) float-adjustment screw, (B) primary float bowl, (C) vacuum diaphragm, (D) main metering body, (E) secondary float bowl, (F) secondary metering block, (G) baseplate, (H) primary metering block, (I) accelerator-pump linkage, (J) idle-mixture screw. There are other components to the Holley, but these are the major pieces. We’re using a new Holley Avenger carb for the photos. Your carb might be a little greasier, but it works the same.

We’ll assume you’ve successfully removed the carb from the engine. Find a drain pan and drain the fuel by inverting the carburetor. This will save you the hassle of spilt gas all over the workbench. Start by removing the primary and secondary float bowls with a 5/16-inch nut driver or a ratchet and socket.

Float-bowl and metering-block gaskets sometimes stick. We use a thin gasket scraper placed between the metering block and the float bowl and then hit it with a hammer to release the gasket. Remove the primary and secondary float bowls, metering blocks, and gaskets from the main body of the carburetor. It’s a good idea to put all the parts in a plastic or metal bin so nothing gets lost or rolls off the bench.

Remove the power valve from the primary metering block with a 1-inch box-end wrench. Also unscrew both small idle-mixture screws from the metering block. The primary and secondary metering blocks and float bowls often look the same. Usually, the primary metering block has a power valve while the secondary block doesn’t. On vacuum-secondary Holleys, the primary float bowl incorporates an accelerator pump, but the secondary bowl does not. Remove the jets with a wide-blade screwdriver to prevent damage.

Remove the accelerator-pump cover. Underneath these four screws you’ll find a rubber diaphragm, a short spring, and a small orange rubber check valve. The check valve allows fuel to enter the accelerator-pump cavity, but when the accelerator lever is pushed, the orange check valve prevents the fuel from travelling back up into the float bowl. Your new rebuild kit should include a new check valve.

This is a good time to loosen the fuel line fittings in each float bowl and remove them. Holleys come with a brass filter element with a spring behind it. If these are present, replace them. Don’t be alarmed if your carb doesn’t have these filters. Many hot rodders eliminate them. We’d suggest using them. If not, you will need some type of inline fuel filter between the mechanical fuel pump and the carburetor.

The needle-and-seat assembly is located under this lock-screw and nut assembly on the top of each float bowl. Loosen the lock screw with a wide-blade screwdriver and then remove the needle-and-seat by unscrewing (counterclockwise) the nut. The needle-and-seat assembly will be replaced in the new rebuild kit.

Turn the carb upside down and look for the rod that connects the vacuum-diaphragm housing to the secondary throttle shaft in the baseplate. There is a very tiny C-clip that retains the rod to the shaft (arrow). Remove this clip and place it in the parts bin. If you are working on a mechanical- secondary carburetor, you can skip this part because there is no vacuum-secondary diaphragm, so don’t go looking for it.

Place the carb upright again. Before we can remove the vacuum diaphragm, we have to first remove the electric choke mechanism. To do this, remove the three Phillips head screws. But before you can remove the choke housing, you have to first remove the small hairpin clip.

The clip is located on a rod that comes down from the choke housing. The long rod fits under the J-shaped plastic piece connected to the backside of the choke housing. Older Holleys used a metal piece here, but it’s shaped the same. Pull the clip and the choke housing will release from the carb. Do not disassemble the choke housing assembly unless it is inoperative.

Now we can unbolt the vacuum diaphragm from the main body with three screws. Since this is a relatively simple mechanism, we won’t disassemble it for this rebuild. This particular style uses a quick-change cover that allows changing the internal spring without removing the whole cover. If you remove the entire cover, be very careful aligning the holes in the diaphragm with the cover screws. The thin rubber diaphragm is very fragile and will tear if the holes are not perfectly aligned. It’s best to leave this alone for now as long as the diaphragm works properly. Check for vacuum leaks.

Turn the carb upside down again and remove the six Phillips head screws that attach the baseplate to the main carb body. Note that there are eight tapped holes with two missing screws in the center of the body. Holley leaves these out to prevent a loose screw from dropping down inside the intake manifold when the engine is running.

Turn the carb upright and remove the accelerator-pump squirter screw and nozzle. Underneath the screw (arrow) will be a small metal needle that will come out when you invert the carb.

Remove the gaskets, needles and seats, power valve, accelerator-pump diaphragm, choke, and vacuum-secondary mechanism from the parts bin. If you are going to soak the carb parts in liquid carb cleaner, then remove the plastic accelerator-pump cam located on the primary throttle linkage.

A gallon of carb cleaner can be expensive and difficult to get rid of since it is extremely toxic. This cleaner works best when you have an extremely grungy carb that may have to soak for a day or so. Otherwise, a can of good carb cleaner (not the citrus stuff) will usually suffice to get rid of the grunge. Afterwards, blow out all the passages with compressed air.

Now you can start the reassembly process. Start by bolting the baseplate to the main body. Holley makes the gasket so that it cannot be installed backwards. Bolt the throttle plate on and place the carb upright and install the accelerator-pump squirter with both gaskets.

Now we can bolt the vacuum-secondary diaphragm housing back on the carb body. Make sure the little cork gasket is in place (arrow). If this gasket is missing or damaged, the secondaries won’t open. Don’t forget to replace that tiny little C-clip on the secondary throttle shaft.

Now install the electric choke assembly. Be sure to place the long rod underneath the choke arm on the choke assembly so the choke will work properly. Then replace the hairpin clip.

Install a new power valve in the primary metering block. Holley again makes it easy to install the metering-block gasket since the pins (arrows) allow the gasket to install only one way. Install the idle-mixture screws in the metering block. Adjust them to 11/2 turns out from full idle.

The bowl gasket will also fit over the pins on the metering block only one way.

First install the new needle-and-seat with gaskets on both sides of the adjusting nuts. Then invert the bowl and adjust the float level by turning the adjustment nut until the float is level. Remember that each bowl screw requires a thin gasket to seal against fuel leaks. Now you can use the screws to install the float bowls.

If you are going to use the internal fuel filters, the spring goes in first, followed by the filter and a small gasket to seal the filter to the fitting. Finally, add a large-diameter gasket to seal the fitting to the bowl.

Install the accelerator-pump assembly in the bottom of the primary float bowl. The spring goes first, followed by the rubber diaphragm, and then the cover. Note that the accelerator-pump lever arm is positioned outboard.

To install the primary float bowl, make sure you line up the accelerator-pump arm with the linkage (arrow). If you forget, you will have to remove the float bowl to reposition the lever under the linkage arm.

Tighten the float-bowl screws and you’re done. Reconnect the carb to the engine and start it up. First check for leaks, then be sure the float level is correct (just touching the bottom of the sight plug), and adjust the idle mixture. Then you’re ready to go!

If there is an icon in the performance world, it has to be the Holley four-barrel carburetor. The beauty of the Holley is that from a plain-Jane 600-cfm to a 1,050-cfm Dominator, all Holley carburetors work the same way. Better yet, its modular design makes the Holley exceptionally easy to work on. Basically, if you can fog a mirror, you can rebuild a Holley. Given that, we’ve assembled a step-by-step rebuild tour through a typical Holley vacuum-secondary four-barrel carburetor. So grab your wrenches and screwdrivers and let’s get started. Oh, don’t forget the fender covers.


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