Mark Houlahan
Tech Editor, Mustang Monthly
December 18, 2013

Demon Carburetors

When Demon carburetors first came on the market, they were offered in several different performance levels from street to race, with varying feature sets applicable to the intended market. A few years ago the company filed for bankruptcy protection and the performance aftermarket looked like it was going to lose a viable source for performance carburetion. Thankfully, new ownership picked up the pieces and not only does Demon offer the original 4150 Holley-style modular Road, Speed, and Mighty Demon carburetors, but it is making waves with a new Street Demon carburetor. The Demon line has been known for its billet metering blocks and baseplate, Idle-Eze feature (adjusting the base idle via a screw in the air cleaner stud socket that allows idle changes without moving the primary throttle plates), and quick-change vacuum secondary pod.

Demons feature four idle mixture screws, one on each side of the primary and secondary metering blocks

Some Demon models feature a choke horn but do not come with a choke plate (see mechanical secondary model in previous photo). If your application calls for a choke to air in cold starting, Demon offers an electric-choke conversion kit that the end user can easily install.
On Demon’s vacuum secondary offerings, the quick-change diaphragm spring design allows for tailoring the opening rate of the secondary throttle shaft with the simple change of a spring.
The Demon’s metering block is made from billet aluminum and houses the jets, power valve, idle mixture screw, and more. The metering block is where the carburetor’s fuel is metered and mixed with air. You can see the various components in this call-out diagram. A) Idle wells B) Main wells C) Main well exit D) Idle-mixture adjustment screw E1) Transfer slot discharge E2) Idle mixture channel controlled by adjustment screw F) Emulsion bleeds G) Power valve
On the opposite side of the metering block you’ll find the main jets. The main jets of the carburetor are a fuel-control orifice and are numbered by size. Changing the jets will affect wide-open-throttle power. Jets are available in pairs or you can purchase a tuning kit with several sizes; perfect for dyno or track tuning.
This shot of the power valve removed from the metering block shows its construction, which includes a diaphragm, check valve, and spring. The power valve gives the carburetor additional fuel in low-vacuum conditions (such as full throttle load). You will find power valves available in different vacuum ratings to aid in tuning the carburetor further.
The Demon’s accelerator pump is found on the primary fuel bowl only, unless it is a mechanical-secondary carb, which has one on the secondary fuel bowl as well. The accelerator pump gives the carburetor a “shot” of fuel on initial acceleration to prevent stumble/lean condition. The accelerator pump can be adjusted via the pump arm adjustment screw and additional tuning is made possible with larger pump capacities and different nozzle orifices.
Mentioned earlier in our opening paragraph on the Demon line, the Idle-Eze function allows base idle speed changes to be made to the carburetor without disturbing the primary throttle blade angle, potentially disturbing the carburetor’s vacuum signal for the transfer slots.
Demons feature four idle mixture screws; one on each side of the primary and secondary metering blocks. Adjusting the idle mixture screws takes a light touch (often just a 1⁄8 to ¼ turn) and should be performed with a vacuum gauge. You want the highest vacuum reading possible, moving from one screw to the next.
All Road, Speed, and Mighty Demon carburetors feature center-hung floats. Ensuring dry float measurements are correct using a float gauge or specific drill size. On-car adjustments can be made using the float adjustment nut on each fuel bowl.
Demon’s newest carburetor is its Street Demon and it takes cues from the original Carter AFB design. The one-piece main body includes a fuel bowl with the top cover gasket above the fuel line to virtually eliminate leaks. The carburetor is available in 625- and 750-cfm models in burnished or hand-polished finish, and with an optional polymer main body. Like other Carter-based carbs, it features a dual bolt pattern to work on square- and spread-bore intake manifolds.
Unlike the traditional “four-barrel” carburetor, which features four individual throttle bores, the newest Demon features a single secondary plate called the Goggle Valve. The Goggle Valve is mechanically connected to the primary throttle valves via a progressive linkage that starts opening at 30-degrees of throttle. The air valve located over the Goggle Valve (seen in the previous picture looking like a choke plate) is vacuum operated like a vacuum-secondary throttle in a modular carburetor, and it is easily tuned via a spring tension screw on the side of the carburetor.
Demon Carburetion offers these handy tuning kits that include metering rods, step-up springs, and primary and secondary metering jet sizes. The metering rods and step-up springs are easily accessed from the top of the carburetor for tuning purposes, while the Street Demon’s jets are accessed by disassembling the carb. ▶
Unlike a traditional modular carb, the new Street Demon uses metering rods and step-up pistons with springs for fuel enrichment instead of a power valve. The metering rods move via vacuum signal and restrict flow through the carburetor’s jets.