Once the C-clip is removed, slide the lever off its pivot pin. The accelerator pump throttle lever arm pivots on this pin. The discharge nozzle, on either the primary or secondary side, is secured with a single Phillips-head screw at the top of the main body between the front two barrels and between the rear two barrels on a Double Pumper.
Hold the discharge squirter steady and remove the screw. One small, thin metal sealing washer is under the screw head and one is at the bottom of the nozzle unit.
Under the discharge nozzle, inside the fuel passage, is a small pump discharge needle valve. Turn the main body upside-down to let this needle valve drop out. This nozzle is being removed from the secondary side of a carb. The discharge nozzle has two small, thin metal gaskets, one at the top between the screw head and nozzle and one at the base between the nozzle and main body. You can easily retrieve it using a small pencil magnet. If you drop it into the venturi, simply open the throttle plate and allow it to drop onto the workbench.
Step 3: Locate Discharge Nozzle Access to the discharge nozzle on the primary side of a carb with a choke is somewhat limited because of the choke plate. Be patient and use a screwdriver to unthread the discharge nozzle screw. Use needle-nose pliers or a hemostat to retrieve the nozzle and screw as an assembly. The easiest method is by carefully turning the carb upside-down onto a clean rag on the workbench.
If you do not remove it now, you may lose it the next time the carb is turned upside-down. To keep things organized, bag the discharge nozzle, screw, gaskets, and needle valve together. Main metering jets have a straight driver slot. A flat-blade screwdriver may be used, but a much better tool choice is a dedicated jet driver such as the one offered by Holley. This tool has a male tang that engages the slot and a centering tip that keeps the tool centered on the jet, which eliminates the chance of the tool slipping off the slot during removal or installation.
Engage the tool and rotate it counterclockwise to remove the jets. Although a flat-blade screwdriver can be used to remove or install main jets, a much better choice is a dedicated jet driver such as those offered by Holley and a few other tool makers. This tool has a center round dowel that automatically centers the tool into the face opening of the jet; the flat driver seats into the jet slot.
The tool prevents slipping, which can occur with a common flat-blade screwdriver. The power valve is threaded into the metering block from the rear. The body of the power valve has a four-sided flat drive instead of a hex. A 1-inch wrench is required for removal or installation. The flats on the power valve are fairly shallow. Although a conventional open-end, box, or socket wrench may be used, a superior choice is a dedicated power-valve wrench.
This is a billet aluminum tool that is specifically designed to perfectly engage the power valve without the danger of slipping off or gouging the flats or nearby surfaces. Rotate the power valve counterclockwise to remove it. The power valve includes a sealing gasket.
If you plan to reuse the existing power valve, make note of the gasket style open center or center with three tangs because you must use the same style of gasket for the power valve. Step 1: Remove Power Valve Power valves have a four-sided flat, requiring a 1-inch wrench.
However, the drive flats are fairly shallow, so care must be taken to make sure that the wrench remains fully seated. The billet aluminum power-valve tool is designed specifically for servicing power valves. Insert the tool onto the power valve, holding the tool flush with the metering block and rotate the tool counterclockwise. The knurled grip provides excellent finger traction.
The white plastic overflow whistle is located on the upper area of the metering block if so equipped and is held in its metering block slot with a single interference-fit pin. To remove the pin that secures the plastic overflow whistle, insert a flat-blade screwdriver into the rear of the cavity and pry upward to push the screw up. The self-tapping screw is tiny, so bag it for reference.
Once the screw has been pushed upward, you may be able to wiggle it free with your fingers. If not, use needle-nose pliers or a hemostat. Step 2: Remove Overflow Whistle After the locating pin has been removed, you can easily pull the overflow whistle from the metering block. The needle screw is sealed with a small round gasket in the metering block. You can remove it using these two simple steps. Rotate the idle-mixture screw counterclockwise for removal. If you plan to clean the metering block with carb solvent, remove the small sealing gasket and replace it with a new gasket.
A tension washer maintains pressure on the housing to prevent the housing from accidentally rotating. An eyelet on the flat-wound spring inside the choke housing engages the choke plate lever. Pull the choke housing straight off; avoid cocking it during removal to prevent potential spring damage. Before removing these screws, remove the small hairpin that secures the vertical choke rod to the choke lever assembly.
Use small needle-nose pliers or a hemostat to remove this pin. When removing the base, note the small orifice with a small round gasket on the backside of the base. This influences manifold vacuum, which circulates air through the housing.
In turn, the pull-off piston activates inside the base and aids in opening the choke plate after the engine warms up. Inspect this small gasket. A small hairpin secures the choke rod to the choke assembly lever. Remove the three screws and the black spring housing along with the spring-steel three-hole washer. The large flat gasket should remove easily, unless heat and age have caused it to stick. If so, use a razor blade to scrape the gasket free.
A tang connects to the choke spring eyelet upper right inside the casting. This spring inside the choke housing is in its rest position. It pushed the choke plate rod up and caused the choke plate to close. As the spring heats, the lever pulls the choke rod down and opens the choke plate.
Needle-nose pliers or a hemostat makes this easy. Using a hemostat allows you to not only grab the pin, but because a hemostat locks, the pin is securely retained during removal.
A hemostat includes a serrated locking tang between the finger rings. To free the spring clip from its grip simply squeeze the tool together and dislocate its locking serrations. A mechanical choke assembly attaches to the main body in the same manner as an electric choke. Remove the pin before removing the three mounting screws. Remove the three screws holding the cast choke base assembly to the main body. Note the air vent tubes on the base casting at left. The smaller tube just inside the round base is a vacuum port that feeds from manifold vacuum to assist the pull-off piston that aids in opening the choke plate when the engine warms up.
A small gasket seals the vacuum port that provides vacuum to the pull-off piston inside the choke housing base. Regardless, before reinstalling the base, always install a new gasket in this location.
Locate the three screws that mount the choke lever base on a series carb to the main body. Before removing the three choke lever assembly screws, note that the choke plate-actuating rod engages to a lever on the bolt-on assembly. The choke rod is secured with a small hairpin that must be removed first. Use a Phillips screwdriver to remove the three choke lever assembly screws from the main body. Remove the choke lever mechanism. The formed bracket at the rear left side of photo shares two of the mounting screws.
This bracket includes a cable pinch mount for the manual-choke cable. The choke plate rod has a nylon plate that helps to guide the rod with reduced friction. Of course, the choke plate could be removed, but that presents a problem because the choke plate is staked to the choke plate shaft. The choke assembly base hides access to the forward-most screw, so the choke assembly must be removed first.
A single Phillips-head screw secures the bottom of the vacuum secondary actuator rod that connects to the secondary throttle plate shaft. Remove this screw first. This lever is engaged to the shaft via a two-sided flat design and the fit may be fairly tight. If necessary, use a small flat-blade screwdriver as a pry tool between the baseplate and lever.
Pry gently, little by little, on opposite sides of the lever until it is free. A small gasket seals the vacuum feed on the backside of the vacuum housing. Remove these four screws and lift off the lid and spring. Always use a new lower gasket and new upper metal washer. When the screw is tightened, the metal washer deforms into a concave shape to seal the countersunk mating between the screw head and the top of the discharge nozzle.
This eases installation and helps to prevent the gasket from slipping. Position the upper metal washer over the screw and then install the screw; tighten it to 30 to 50 in-lbs. Make sure that the center notch on the discharge nozzle body seats into the small pocket on the main body.
A choke mechanism incorporates a plate that closes over the primary side to create a stronger vacuum signal to restrict air and allow a richer fuel mixture to help a cold engine start. The choke plate remains closed or nearly closed to increase fuel atomization until the engine warms up. This vacuum-assisted choke also has a rubber diaphragm that opens the choke a little as engine vacuum begins to develop when the engine starts; it helps to prevent an over-rich condition.
The choke plate works in tandem with a fast-idle cam that increases engine speed during this warm-up period. Most Holley carbs with a choke use either a mechanical or an electric choke.
The mechanical choke is controlled directly by the driver via a cable. The electric choke is fitted with a thermostatic spring inside a housing; careful adjustment provides dependable cold-engine starts.
Before installing the choke housing, rotate the spring-loaded choke arm so that the small arm rests against the actuator pin shown.
As you install the housing into the main body, the hole in the long arm must engage onto the choke-plate rod. Secure the choke-housing screws to the main body, making sure that the choke-plate rod is engaged in the lever arm. Torque the screws to 6 in-lbs. The choke-plate rod protrudes past the spring-loaded choke-housing lever. A cotter clip must be installed to the hole in the end of the rod. Install the hairpin clip onto the choke-plate rod.
Slide the electric choke until the choke plate rod is captured by the fast-idle cam. You then need to secure the rod to the arm. A vacuum port is at the rear of the electric choke housing. Be sure to install a new cork-rubber gasket. Install a new gasket between the electric choke housing and cap. Make sure that the eyelet on the wound spring engages the lever tang in the choke housing when you install the electric choke cap.
Install the spring washer onto the choke cap and lightly secure the three mounting screws in the cap. Initially, adjust the choke cap index at the center mark on the choke housing. The choke setting can be fine-tuned on the engine.
The vacuum secondary system contains a vacuum housing with a rubber diaphragm and spring. Assembly is a bit easier if the housing is held upside-down while installing the spring, diaphragm, and housing cover. It may be difficult to avoid a vacuum leak while installing the secondary diaphragm into its housing. Push the rubber into its proper shape, with the gasket surface flat where it contacts the housing.
This helps to flatten out the rubber. Notice the small vacuum orifice at the upper right. The captured hole in the gasket must align with this orifice. Push the diaphragm into the cap while you hold the cap, spring, and diaphragm upside-down on a flat surface this keeps the rubber flat. While holding the cap and diaphragm, slip the housing over the rod and onto the rubber, aligning the screw notches of the gasket to the screw holes in the cap and housing.
With the cap secured by the four screws, examine its perimeter to see if the gasket has pushed out. If all looks good, you can perform a vacuum check. Be sure to install a new cork-rubber gasket in the vacuum port. While holding a finger over the vacuum port, push the rod into the housing until it is fully seated; release the rod while still plugging the vacuum port.
If the rod drops freely, you have a vacuum leak and must re-position the diaphragm. Install the secondary vacuum assembly onto the main body with its three screws. Slip the eye loop of the vacuum rod over the pivot lever. Install the tiny C-clip to secure the bottom of the operating rod; using a hemostat can make this a bit easier. The clip is very small, so be sure to handle it over a clean work surface in case you drop it. Verify that the C-clip is fully seated in its groove. The following is an overview of the procedure for installing a throttle plate.
Step 1: With the throttle-plate screw holes aligned with the shaft, install the screws. Add one drop of Loctite to each screw thread. Snug down both screws, and then back them out half a turn. Lightly open and close the throttle. Do this at least six times so the plate can seat in the throttle bore. Step 2: Tighten the screws. Open and close again to check for binding. If any binding is present, repeat Step 1. If there is no binding, repeat with the next throttle bore. Step 3: After the new throttle plates are installed, open the throttle to wide open and release.
Do this as fast as possible at least 10 times. Be absolutely sure that there is no binding present. Repeat all steps if necessary. A secondary metering plate used on the secondary side of carbs has drilled restrictions rather than a metering block that has replaceable jets. Holley offers a wide range of metering plates for tuning carburetors.
Clutch-head screws are used because the drive style is positive and not prone to stripping out, as long as you use a clutch-head tool. I prefer to use the style Holley uses, but some Demon carb metering plates are secured with Phillips-style screws, so either works. Sets of new clutch-head screws along with plate and bowl gaskets are available as a kit if you destroy or lose any of the original screws.
Carb studs should be hand-snugged to the intake manifold. Torque the nuts only to 60 to 80 in-lbs. Do not overtighten the studs to the manifold. Overtightening the studs to the manifold can result in slight splaying. Only apply the torque value to the nuts that engage to the studs; do not torque the studs to the to 80—in-lbs value. Once the nuts are torqued to specification, the studs are secured. This is a common mistake among novices. Never excessively tighten any stud. If you liked this article you will LOVE the full book.
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