Look under the hood of a hopped-up classic truck and you're likely to find an engine topped with a four-barrel carburetor. Thanks to the flexibility of a pair of primary venturis for low to mid-rpm operation, and a secondary set to handle the engines wide-open-throttle needs, four-barrels are certainly the most popular type of carburetors, and some of the best come from Barry Grant and Holley.Similar in construction, both manufacturers offer carburetors in a variety of cfm ratings with vacuum or mechanical secondaries, with single or double accelerator pumps, and they use power valves for enrichment systems. While the architecture is similar, there are subtle differences in the construction of some components, but for our purposes, we're going to look at them together. The rebuild procedure is based on a Barry Grant Demon, while the tips are from Holley, but the following information is applicable to both.
Demon Carburetor Service
By Sam Moore
Rebuilding the carburetor begins with the simple procedure of removing the air cleaner, disconnecting the throttle linkage, the fuel and vacuum lines, and loosening the four nuts and washers that secure the carburetor (and, if fitted, disconnecting the transmission kick-down mechanism). The tools required for refurbishing the carb are: a 5/8-inch wrench (for the needle-and-seat), a 1-inch wrench (for the power valve), a Phillips head screwdriver (needle-and-seat), and either a 1/4-inch Allen wrench (float bowl retaining bolts, Demon) or 5/16-inch nut driver (Holley).
Once dismantled, wash the components and clean the orifices (especially the air bleeds) with carburetor cleaner and compressed air. Replace the old small parts with new ones, including good quality, red-colored, non-stick gaskets, the accelerator-pump diaphragms, the needle-and-seat assemblies, and so on. Adjust the floats to be approximately 0.450-inch from the top of the bowl and in line with the bowl-screw bosses when the bowl is turned upside-down. This preliminary step is a "dry" setting; the float-level settings will receive final adjustment when the carb is installed and the motor running.
Next, adjust the idle-mixture screws to their initial setting, which is 1 1/2 turns out from their bottoming position. Also check the position of the throttle plates relative to the transfer slots. The throttle plates (butterflies) and transfer slots are located in the bores of the baseplate, and should be adjusted such that only 0.020-inch of each slot is visible. Incorrect adjustment of the transfer slots is one of the most common causes of poor idling. If too much of the slot is exposed, the idle-mixture screws cannot control the quality of the idle.
Using the new baseplate gasket, refit the carburetor and connect the linkages and lines. Start the engine and bring it to operating temperature. Adjust the idle-speed screw (located on the left-hand side of the baseplate--it adjusts the primary throttle shaft) to provide the desired idle speed. Slowly turn the idle-mixture screws in or out as necessary to establish the best idle quality. Conduct this procedure twice. The first attempt will provide an approximate adjustment; the second will accomplish a finer setting. Always follow the tuning instructions provided by the manufacturer of the carburetor (for the Demon, the data can be downloaded from www.barrygrant.com).
Final float level adjustments of a Demon carburetor on a street-driven application are simply made by setting the fuel level to the 1/4 distance of the viewing window while the engine is idling. To check float levels on carburetors without sight glasses, remove the float-level plug, and ensure the outpouring fuel doesn't create a fire hazard on a hot manifold. Fuel should barely trickle from the primary or front end of the carburetor and be slightly higher at the rear. When turned clockwise, the hexagon nut on the needle-and-seat assembly will adjust the float downward, and upward when turned counterclockwise. To adjust the needle-and-seat, loosen the screw in the middle of the assembly very slightly and retighten when adjustments are finalized. By changing the float levels, the amount of fuel in the bowl and the carburetor's ability to feed the main jets is altered. By increasing the height of the float levels, the engine's response is quickened. This is an adjustment that is frequently used to eradicate a lean stumble. By lowering the float levels, the activation of the main metering circuits is delayed and consequently causes a leaner mixture when coming off idle. For street applications, the latter condition is more economical. To prevent clogging of the air bleeds, use a quality air cleaner like those from Rush Performance Filters.
Tips On Holley Carburetors
By Ron Ceridono
As we've said, the Holley and Demon carburetors are quite close in design, so the overhaul procedure is virtually identical. However, if rebuilding a Holley carburetor is something that you'll be doing in the future, we suggest you order a Holley Tuning & Troubleshooting DVD from BoxWrench.net (they also offer a huge poster of a 4150 mechanical secondary carburetor and an excellent DVD on basic engine rebuilding).
Some of the most often asked questions about Holley carburetors have to do with power valve selection, idle adjustment, jet selection, and accelerator pump adjustment. Thanks to the tech team at Holley, here is some advice on each.
Selecting Power Valves
The power enrichment system supplies additional fuel to the main system during heavy load or full power situations. Holley carburetors utilize a vacuum-operated power enrichment system and a selection of power valves is available to "time" this system's operation to the engine's specific needs. Most of the popular Holley "Street Legal" and "Street Performance" carburetors incorporate a power valve blowout protection system in the form of special check valve in the throttle body. This check valve is designed to normally be open, but will quickly seat to close off the internal vacuum passage when a backfire occurs.
If you have a carburetor older than 1992 (or you have experienced an extreme backfire) and suspect a blown power valve, use this simple test. At idle, turn your idle mixture screws (found on the side of the metering block) all the way in. If your engine dies the power valve is not blown.
Each Holley power valve is stamped with a number to indicate the vacuum opening point. For example, the number "65" indicates that the power valve will open when the engine vacuum drops to 6.5 inches or below. To determine which power valve to use, an accurate vacuum gauge should be used. A competition or race engine with a long duration, high-overlap camshaft will have low manifold vacuum at idle speeds. If the vehicle has a manual transmission, take a vacuum reading with the engine thoroughly warmed up and at idle. If the vehicle is equipped with an automatic transmission, take the vacuum reading with the engine thoroughly warmed up and idling in gear. In either case, the power valve selected should be half the intake manifold vacuum reading taken. As an example, a 13-inch Hg vacuum reading divided by 2 indicates a 6.5 power valve. If the reading divided by 2 lands on an even number, you should select the next lowest power valve. Example: An 8-inch Hg vacuum reading divided by 2 equals a No. 4 power valve. Since there is no No. 4 power valve, you should use a 3.5.
Idle Adjustments
The initial adjustment is made by turning the mixture screws (located on each side of the main metering block) in a clockwise direction until they lightly hit bottom. Back them both off 1 1/2 turns. Connect a vacuum gauge to a carburetor vacuum port that has full manifold vacuum at idle. Start the engine and once it has warmed up and the idle stabilized, the choke should be disengaged. Adjust the idle-mixture screws to obtain the highest vacuum reading. In some cases it is advisable to turn the idle screws in just enough to get a 25-rpm drop; this is known as lean-best idle and will help keep the plugs clean.
Jet Selection
When changing jets, go two sizes; there is approximately a 4 1/2-percent flow difference from one jet size to the next, and one size won't make that much of a difference. If you must go up or down excessively, eight to 10 jet sizes, there is a problem with the fuel delivery system or the carburetor is wrong for the application.
Accelerator Pump
The first adjustment to check is the clearance between the pump operating lever and the pump diaphragm cover's arm; at wide-open throttle, it should be around .015-inch. The purpose for this clearance is to assure that the pump diaphragm is never stretched to its maximum limit at wide-open throttle, which will cause premature pump failure. With the proper clearance established, operate the throttle making sure that the accelerator pump arm is being activated the moment the throttle begins to move. This will assure that pump response will be instantaneous. Adjustments are made with the screw that is located on the accelerator pump arm.
The amount of fuel that can be delivered by one accelerator pump stroke is determined by the pump's capacity and the profile of the pump cam. The period of time that it will take for this predetermined amount of fuel to be delivered is affected by the pump nozzle size. A larger pump nozzle will allow fuel to be delivered much sooner than with a smaller pump nozzle. If more pump shot is needed sooner, a larger pump nozzle size is required.
During acceleration tests, if you notice that the car first hesitates and then picks up, the pump nozzle size should probably be increased. A backfire (lean condition) on acceleration also calls for an increase in pump nozzle size. On the other hand, if off-idle acceleration does not feel crisp or clean, the pump nozzle may be too large.
Holley accelerator pump nozzles are stamped with a number, which indicates the drilled pump hole size. For example, a pump nozzle stamped "35" is drilled .035-inch. Pump nozzle sizes are available from .025- to .052-inch (whenever a .040-inch or larger accelerator pump nozzle is installed, the "hollow" pump nozzle screw should also be used).
As with main jets, when changing the pump nozzle it's best to jump more than one size; in this case go three sizes. For example, if there's an off-line hesitation with a No. 28 (.028-inch) pump nozzle, go to a No. 31 (.031-inch). If a No. 37 (.037-inch) or larger pump nozzle is used, then a 50cc pump should also be installed.
Once a pump nozzle size selection has been made, the accelerator pump system can be further tailored with the pump cam. Holley offers an assortment of different pump cams, each with unique lift and duration profiles. Switching cams will directly affect the movement of the accelerator pump lever, which impacts the amount of fuel available at the nozzle.
Changing pump cams is done by removing one screw, placing the new pump cam next to the throttle lever, and tightening it up. There are two and sometimes three holes in each pump cam, numbered 1, 2, and 3. Placing the screw in position No.1 activates the accelerator pump early, allowing full use of the pump's capacity. Generally this position is best for vehicles that have lower idle speeds (600 or 700 rpm) because there is a good pump shot available as the engine comes off this relatively low idle. Positions 2 and 3 delay the pump action, relatively speaking. These two cam positions are good for engines that idle around 1,000 rpm and above. Repositioning the cam makes allowance for the extra throttle rotation required to maintain the relatively higher idle setting. Pump arm adjustment and clearance should be checked and verified each and every time the pump cam and/or pump cam-position is changed. CT
1.
Idle circuit fuel enters here from a main jet. The vacuum below the closed throttle plates at the idle discharge orifices and transfer slots draws the fuel upwards through internal idle circuits to the idle feed restrictors. The idle feed restrictors regulate the amount of fuel in the idle circuits.
2.
At the idle feed restrictors, the fuel is introduced to the air that is traveling down from the idle air bleeds above (also drawn by the vacuum below the closed throttle plates). Emulsification (mixing) of air and fuel occurs in this vertical channel as the air/fuel mixture for the idle circuit travels downwards to the idle discharge orifices and the transfer slots, located in the baseplate.
3.
Passageway that leads to a transfer slot. The function of the transfer slots is to provide sufficient air/fuel mixture during the transition between the supply of idle circuit fuel and main circuit fuel.
4.
Passageway that leads to an idle discharge orifice. The function of the idle discharge orifices is, unsurprisingly, to provide sufficient air/fuel mixture during idle. Adjustment of the idle mixture is performed by the adjacent screws, which penetrate these particular passageways.
5.
Power valve aperture. Usually, power valves are considered unnecessary in drag racing applications. They are more suited to cruising and generally work in conjunction with smaller main jets, providing additional fuel when required.
6.
Emulsion bleeds are set in the main fuel circuits. Air is drawn in from the high-speed air bleeds above and mixes with the fuel in adjacent internal passageways behind the emulsion bleeds. The vacuum generated by the boost venturis draws the emulsifying main circuit fuel upward, out through the main well exits at the top of the metering blocks (No. 7), and discharges it through the boosters.
