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Quality Small Parts for Carburetor Service

By Sam Moore

The art of carburetor maintenance. Learn the basics, restore the performance, and enjoy the experience!

Carburetors, generally speaking, don’t wear out, but they can lose their tune and therefore need to be inspected and serviced periodically. The task of rebuilding the ubiquitous 4-barrel is fairly straightforward and one that should be conducted about once a year. The kits of small parts to refurbish the carburetors include carburetor rebuild kits and carburetor service parts. These contain all the necessary gaskets, ‘O’ rings, replacement diaphragms for the accelerator-pumps, and needle-and-seat assemblies, etc. Also, during carburetor servicing, replace the fuel filter element. None of these tasks are complicated, and the results can be especially gratifying as the engine and its attendant fuel system regain their former authority.

Rebuilding the carburetor begins by 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: 5/8” wrench (needle-and-seat), 1” wrench (power valve), Phillips head screwdriver (needle-and-seat), and ¼” Allen wrench (float bowl retaining bolts, Demon, or 5/16” 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, including: good quality, red-colored, non-stick gaskets, the accelerator-pump diaphragms, as well as the needle-and-seat assemblies, etc. Adjust the floats to be approximately 0.450” from the top of the bowl and in line with the bowl-screw bosses when the bowl is turned upside-down. This is a “dry” setting – a preliminary step; the final 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 one-and-a-half-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” 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, bring it to operating temperature, remove the vacuum advance hose from the ported-vacuum connection on the baseplate, and check the initial ignition timing. The initial timing is determined mainly by the rating of the camshaft and its valve overlap. This rating is referred to as the camshaft duration at .050” of lift. The best indicator for initial timing on engines fitted with after-market camshafts can be found at the Demon Selection Guide at www.barrygrant.com. The initial timing for most stock V8 engines with 4-barrel carburetors is around 10° to 12° before-top-dead-center; that is, the number 1 plug will fire at the selected degree setting before the piston reaches the top of its stroke. The amount of initial timing increases in engines with performance camshafts. If the engine fires too late, the charge won’t burn completely and the motor will be rich and lazy and lack power. If it fires too early, the engine will usually show signs of overheating. It can also be more difficult to start or suffer pre-ignition or detonation. Therefore, it’s vital to know the engine’s ignition timing is correct before making the final carburetor adjustments. To reverse their order could adversely affect the carburetor settings.

Adjust the idle-speed screw, which is located on the left-hand side of the baseplate (it adjusts the primary throttle shaft) to provide the desired idle speed. Using a vacuum gauge connected to the constant-vacuum port of the carburetor, slowly turn the idle-mixture screws in or out as necessary to establish the highest vacuum and 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 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 ¼ 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 should 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 counter clockwise. 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 heights 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 reduce the ingress of dirt in the air bleeds, use a quality air cleaner like those from Rush Performance Filters.

Images and Suggested Captions

All photographs courtesy of Barry Grant, Inc.

Cleaning the air bleeds with carburetor cleaner. Air bleeds become clogged from dirt and dyes in the fuel, which can cause hesitation or a high-speed miss in an otherwise perfect carburetor. Rush air filter elements are made of long staple cotton gauze, which maintains high airflow, yet traps the dirt before it reaches the carburetor.

Insist on a quality carburetor rebuild kit. The BG (PN 190004) service kit includes every possible gasket, seal, and ‘O’ ring needed for the job.

Install quality, red-colored, non-stick gaskets.

Needle-and-seat assemblies have a tough life. Replace them during servicing.

Replace the accelerator-pump diaphragms to ensure that the squirters function properly. Carburetors with secondary throttle plates that are operated by vacuum have one diaphragm, while the secondaries of the mechanically operated carbs have two.

See the vital positioning between the throttle plate and the transfer slot. As the throttle plates open, the transfer slots are uncovered and provide the fuel for the initial surge of air entering the manifold. At idle, however, the transfer slots must not be uncovered more than shown.

Incorrect float levels allow the carburetor to either flood or to run out of fuel. Float levels are subject to two settings: the initial factory setting and a final adjustment once the carburetor is mounted on the manifold with the engine running.

The fuel pump’s best friend - the indispensable element of the fuel system. The BG 5000 fuel filter is light, compact, and its replaceable element filters to 8 microns. They can be used with gasoline, alcohol, or nitro. The BG Belt-driven fuel pump and the Gerotor-style of the continuous-duty Mighty Enduro and Sumo pump operates under very close tolerances; so close, in fact, that with poor filtration the ingress of one grain of sand can destroy them! Renew the fuel filter element at least once a year. Ensure no foreign particles fall from the element into the canister during renewal, and that the internals of the canister are spotless. Always have the fuel filter placed ahead of the fuel pump.

The ported vacuum and constant vacuum channels with their two, black-capped connections can be seen on the baseplate of this Demon carburetor. The inner machined channel that leads to the throttle bore is a source of constant vacuum, which is used to power the automatic transmission downshifting modulator and any accessory requiring vacuum. The outer machined channel is part of the ported vacuum system, which provides the energy to operate the distributor’s vacuum advance mechanism.