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).