The Mighty Demon
The basics of Barry Grant’s Stout Carburetor
Story and Photos by Sam Moore

The modular carburetor is a remarkable instrument. Its chief attributes lie in its capacity to emulsify (mix) air and fuel and to atomize it (convert the air/fuel mixture to very fine particles), its ease of adapting to change, and its inexpensiveness.

Even today, more than 100 years after its invention, its capacity for emulsifying and atomizing is impressive and has not been surpassed by fuel injection. Further, if one considers the simplicity of re-tuning the carburetor following a camshaft change compared with the irksome re-mapping of a fuel-injection system, there’s no contest – the simplicity of the carburetor wins every time. And when compared to fuel injection in terms of cost, it’s the performance bargain of the century. For these and other reasons, the popularity of the venerable fuel-metering device continues; even Detroit is increasingly offering their crate motors with carburetion.

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Since it’s introduction in 2002, street rodders have embraced Barry Grant’s Mighty Demon carburetor. With mechanically operated secondary throttle plates, it was developed for high-performance street rods, muscle cars, and high-output crate motors. Consequently, it’s ideally suited to engines with camshaft profiles of up to 260 degrees of duration at .050" lift, large cylinder heads and intake manifolds, and higher compression ratios. The Mighty is a sophisticated device and can be identified by its billet metering blocks with four-corner idle; its billet baseplate with Idle-Eze; replaceable air bleeds and idle-feed restrictors; as well as those large sight windows on the fuel bowls. In common with other modular-style, four-barrel carburetors, it comprises four main components: the main body, two metering blocks, two float bowls, and a baseplate.

The Main Body and its Essential Features

Concentracast is the casting technique used to produce the main body of the Mighty Demon carburetor. The chief objective of this process is to generate robust airflow and to achieve uniformity during the casting process. Its effect is especially noticeable in the smoothness of the air entries and the concentricity of venturi bores (round and ridge-free). Vigorous airflow stimulates effective emulsification in the metering circuits and provides the capacity to atomize a dense, combustible charge on its path to the combustion chamber. As standard equipment on all Demons, the main body of the Mighty is fitted with Helicoil inserts. The purpose of the Helicoil is to improve the attachment of both the metering block and float bowl to the main body. Both the bowls and the main body are cast in a material known as Zamek 3. To combat vibration, the venturi boosters are formed in ZA 8. Because working carburetors are unavoidably exposed to heat cycles (alternating hot and cold periods), the float-bowl retention bolts need to be tightened periodically.

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Metering Blocks – Form and Function and Cast Zinc versus Billet

Through the various ports and internal channels of the metering blocks, fuel is introduced from the float bowls and air is drawn downward through the eight air bleeds on top of the carburetor into the internal passages where the emulsification process begins. The resulting air/fuel mixture is dispersed via the main circuits and the idle circuits into the air stream where it becomes atomized on its journey downward into the cylinders. Though cast zinc metering blocks are still in evidence, Demon Carburetion prefers the higher quality, solid aluminum block as it overcomes the risk of porosity developing within its interior. Obviously, a cross-sectional analysis emphasizes the solidity of the billet component compared to the potentially porous nature of its cast counterpart (a fine line of porosity penetrating the metering circuits would seriously undermine performance). These billet-metering blocks also feature idle-mixture control screws on all four corners. Their purpose is to provide maximum adjustability of the idle circuit. Because billet metering blocks are subjected to drilling, the metering passages of the Mighty Demon are cleared by compressed air and flushed prior to flow testing to remove any drilling debris. Subsequent, fine alloy dust particles in the metering passages are of little consequence.

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Fuel Bowls – the Highs and Lows

Maintaining the fuel at the proper level in the fuel bowl is the responsibility of the float. A correct float level allows the metering circuits to function properly. A higher float level enrichens the mixture while a lower level makes it leaner. The sight glasses on the Mighty fuel bowls permit safe, quick, float-level adjustments without risk of fuel spillage. Float levels are subject to two settings: the initial factory setting and a final adjustment once the carburetor is installed with the engine running. To check the initial setting, the fuel bowl is removed and the distance between the float and the roof of the bowl measured; the gap should be 0.400". It can be measured with either a ruler of a caliper. The final float-level adjustment should be conducted with the car on a level surface, the engine warmed to normal operating temperature, running at normal idle speed, and with fuel pressure not exceeding 7 P.S.I. On the float bowl of the Mighty Demon there are three horizontal marks beside the sight-glass port. Under normal circumstances the center mark indicates the preferred float level. If necessary, adjust by slackening the center screw and rotating the outer 5/8" hexagon nut. Counter-clockwise will raise the float level; clockwise lowers it.

Baseplate with Idle-Eze

The baseplate of the Mighty Demon carburetor serves two principal functions: to connect the carburetor to the engine and to house the throttle plates (butterflies) that control intake. The baseplate also contains the idle-mixture circuits, the transfer slots, and the IdleEze mechanism -- an innovative device incorporated in the Speed and Mighty models at the beginning of 2004. The IdleEze is a spring-loaded, needle valve and brass seat assembly, which is located in a threaded hole in the center of the baseplate. Its purpose is to introduce and meter an additional source of air to the intake plenum. It’s effective and useful on applications where a performance camshaft has been fitted, especially at idle and off-idle conditions.

Carburetor Selection and Valid Ignition Timing Information

The Mighty Demon is produced in five forms: 650, 750, 825, and two 850 models; one supplied with annular venturi boosters, the other with the downleg style. They are also prepared for supercharger use and have a boost-referenced primary power valve and a fuel curve calibrated accordingly. Selecting the right model requires knowledge of engine displacement, camshaft profile, compression ratio, design of intake manifold, weight of vehicle, type of transmission, stall speed, and gearing. To begin the selection process and to become acquainted with the initial ignition timing requirements for tuned engines, view the carburetor selection recommendations at www.barrygrant.com 

Tuning and Checking the Tune with Modern Apparatus

Tuning basically means providing the engine with the most favorable air/fuel ratio throughout the rev band. The first step in tuning, however, is not to focus on the carburetor, but instead to check the vehicle’s initial ignition timing; to do otherwise is unproductive and undermines the potential of the carburetor. If the ignition timing is retarded the engine will not burn the fuel efficiently. Conversely, if the timing is too advanced, the engine will run hot which introduces potential for detonation and damage. Use a timing light that is known to be accurate and ensure that when the number one piston is on top-dead center the harmonic balancer on the front end of the crankshaft is in alignment with the zero marker.

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The carburetor tuning process begins by establishing the position of the throttle plates (butterflies) in relation to the transfer slots before the carburetor is installed. With the throttle plates in the closed position, only 0.020" of transfer slot should be visible (the slot should give the appearance of a small, square shape). Before installing the carburetor on the intake manifold, verify the initial setting of the float as mentioned previously. With the engine running at normal operating temperature, adjust the idle-mixture screws until the engine reaches its optimum idle. Clockwise adjustments cause the mixture to become leaner; counter-clockwise richer. The adjustments should be made in small increments of approximately ¼-of-a-turn at a time. Once the idle mixture has been set, adjust the Idle-Eze to obtain the desired RPM by engaging it with a screwdriver through the air cleaner stud hole. Changing main jets is usually unnecessary unless there’s a 30-degree fluctuation in ambient temperature or an altitude change of 1200 feet.

Of carburetor tuning troubles, probably the most common is the "off-idle stumble" experienced during rapid acceleration. Assuming the float levels are set correctly, the simplest cure for this difficulty is to increase the size of the squirter. The squirter’s function is to supply an additional shot of fuel during transition from idle to main – a supplemental shot to bolster the fuel supply during the vital moments when the idle circuit has given its best and the main circuit is about to be activated. The orifice of the squirter nozzles fitted to the Mighty Demon as standard is 0.031" in diameter. However, larger tuning replacements are available in increments of 0.002" to 0.003". The immediate sizes above standard are 0.035", 0.037", 0.040", 0.042", and increasing to 0.052". When repairing an off-idle stumble by squirter-nozzle tuning, increase the nozzle by one size at a time.

Now that they’re affordable, the use of a wide-band oxygen sensor seems the easier approach to check air/fuel ratios. The Innovate LM1 is reputed to be the world’s first fully digital air/fuel ratio meter and is capable of capturing and storing up 44 to minutes of AFR data, which can be downloaded to a PC. The benefits are as follows: if the meter indicates poor air/fuel ratio at idle, correct it by adjusting the idle-mixture screws. A poor reading under acceleration, as discussed above, would indicate a change of squirter or, perhaps, the accelerator pump cams. On the other hand, if a part-throttle cruising problem was identified it might be resolved by checking the engine’s vacuum and adjusting the rating of the power valve. The LM1 air/fuel meter won a SEMA Best New Product Award at last year’s trade exhibition.

Maintaining the Mighty Demon with Filtration and the Occasional Rebuild

The carburetor is an air/fuel metering device which requires some maintenance. Promote reliability and longevity with good filtration. Employ a large diameter air filter of, say, 14" by 3" or 4" deep. If clearance is tight, contemplate the new low-line, pannier-style hotrod air filter from Rush Performance Filters. Also, periodically wash and clean both the idle and the high-speed air bleeds with carburetor cleaner and compressed air. Situated on top of the carburetor, they are eight in number and if they become blocked, or partially blocked, they’ll overly enrichen the Mighty’s air/fuel ratio.

Exploit the fuel filter to its fullest. They are designed to protect fuel pumps, fuel-pressure regulators, carburetors, and nitrous-oxide systems from dirt and debris. They should not restrict or impede fuel flow and should be directly placed before the fuel pump. The most popular fuel filter associated with block-mounted fuel pumps is the compact, commonly available, and inexpensive In-line style. But for performance engines with electric or mechanical fuel pumps containing gears, a larger In-line style, typically 11" long x 2" diameter, or a canister type is necessary. Also, the latest Dry-break device boasts maximum filtration with minimal flow loss; a washable, fine-mesh stainless steel filter; and a dry-break valve that prevents fuel from gushing out of the housing during inspection or element replacement. The Dry-break and the canister style attach directly to the inlet of the fuel pump; the In-line style is usually attached to a chassis rail or similar.

Every few years, remove the carburetor and renew the gaskets, O-rings, replacement diaphragms for the accelerator pumps, and the needle-and-seat assemblies with high-quality small parts. Servicing of the carburetor small parts is dependent upon conditions; tough conditions, i.e. salty air, dust, dirt, and dampness demand shorter periods between servicing. Correctly tuned and maintained, the Mighty Demon will perform beyond expectations.