Tech Video: New Street Demon Carburetor Makes Installation Simple

streetdemon-leadartWhen it came time to bring our fastback out of its cocoon, we found that the years of storage had taken it’s toll on the carburetor. We opted to install something fresh on the engine by way of a new Street Demon carburetor from Demon Fuel Systems.


One of the best engines Ford ever produced, the 289ci 271hp K-code engine has been modified.

The Holley was a great carburetor from a technical standpoint, and worked well when the car was running. When we added it up, we figure the Holley doesn’t have much more than about 50 hours of use on it.

But, the Holley does have a few internal problems due to years of neglect. Namely, it’s full of varnish because the fuel sat too long in the bowl. That will render almost any carburetor ineffective. A complete rebuild will mean gaskets, new needle and seats, and a thorough going-through to salvage drivability.

The Holley sat on top of a Weiand aluminum intake and they were installed way back in early 1991. We also swapped in a Wolverine hydraulic cam (204/214 at .050, .448/.472 lift), but then the car got packed away in storage and there it sat for the past 20+ years.

Street Demon Carburetor

625CFM Street Demon – Part #1901

  • Three finishes: polished, burnished, polymer
  • Two sizes: 625 and 750 CFM
  • Patented Goggle Valve Secondary (GVS)
  • Ready-to-run installation
  • Electric choke
  • One-piece air horn
  • Available TPS kit allows use with 4L60 and 4L80 transmissions

In that time, however, the fuel that was in the carb turned into something that could have patched the holes in the Titanic.

It wasn’t a question of a design flaw with the 4010, but rather simple neglect. Add in the fact that the car was stored in South Dakota, where drastic temperatures range from -25 up to 105, and you have a recipe for disaster.

The last time the car was brought out from the back of the garage and we tried to start it the troubles began. It simply didn’t want to run…or idle smoothly. It received some attention, but needed much more.

Worse still, the high concentration of ethanol that had been mixed in the fuel got to the gaskets and degraded them to the point where those, too, were a dangerous problem.

When we did get the car to run, we noticed fuel leaking on the manifold. We’re not surprised at that, but one thing that you don’t want happening to your classic Mustang is to have it burn to the ground because of a four dollar gasket.

The Demon has an electric choke for cold weather starting, and a single fuel inlet to the integrated fuel bowl for simple fuel line connections.

Demon Carb Installation

We decided to swap out the older carb for a new 625 CFM Street Demon from Demon Carburetors. The all new design from Demon had us curious, as did the patented and technical features on the Street Demon. It’s not your typical carburetor, and we could tell that just by the looks of it.


Care should be taken whenever removing a carburetor that nothing falls into the open intake manifold.

In many ways, the install was a smart and easy fix. Swapping out the carburetor took only about an hour, and the result is that the new Demon has the 289ci 271hp K-code engine humming better than it has in a long time. It started up quickly and idled smoothly but still coughed and lagged when when the throttle was stomped. (More on fixing that later).

There is little doubt that the venerable Holley is a classic carb and one that has been part of performance lore since its inception, but there have been a few design enhancements since the Holley came out of its box. This is especially true in the areas of materials and production techniques used to produce the Demon. The new Street Demon has taken advantage of them all.

Back when the Holley was new, aerospace engineering meant putting guys on the moon. Aluminum was the basic material used (which is good), but computers were something that took up an entire room and only used by the government to, again, put guys on the moon (which was pretty good, too). Composites meant plastic, and ceramic was something that you drank your coffee from.

The latest Demon carbs use technology that wasn’t even invented back in the day. Modern composite materials, computer simulations, more accurate flow characteristics and dyno testing has taken what was the black art of carburetor tuning and made it into something that is easier than ever to accomplish.

Demon supplies all necessary gaskets with their carb, making installation all that much easier. A rotating pattern should be used when tightening up the mounting nuts so that the carb has consistent stress on all four corners.

Thanks to the multiple exterior finishes of Hand-Polished, Ball Burnished, and (as in this case), the Polymer with Burnished Aluminum, there is a Street Demon that would look good on any engine.


The stock throttle-actuating arm actually fits into the Demon’s linkage, but is a tiny bit loose. The owner says that he will look for a nylon bushing to take up any play.

We chose to go with the Polymer fuel bowl as Demon claims that it offers significant heat insulating benefits over the aluminum fuel bowl, and keeps fuel up to 20º cooler for optimum performance with modern fuels.

But when it’s time to put your foot down and make horsepower, the Demon is right there, too.

Getting a carburetor to meter correctly at all throttle positions has always been the Holy Grail for tuners. The Street Demon accomplishes this by using small(ish) 1-3/8-inch primary throttle bores with their “Triple Stack” boosters to deliver crisp, off-the-line throttle response and low rpm drivability. They also provide improved fuel economy, which has become a major factor since the days of the older Holley that was being replaced.

Demon has developed and even patented what they call their “Goggle Valve Secondary” (GVS) throttle plate. The patented GVS delivers over twice the airflow afforded by the primaries, so big power is there when you want it. Adjustments to the secondaries is a simple turn of a screw to tighten or loosen spring tension.


The patented goggle valve secondary is aptly named, and delivers more than twice the air flow of the primary bores.

The new carb uses a torsion spring valve control that, according to Demon, permits a seamless transition from the primary to secondary response. It’s designed to reduce lag that we’re all used to when the secondary’s takes over from the primaries thanks to a unique contoured profile that Demon says provides better fuel distribution to the cylinders.

When the Mustang first asked the secondaries to open on a deserted road, the spirited little V8 fell on its face. Demon recommended adjusting the torsion a half turn of the screw to slow down the secondary air valve’s opening rate and make the transition much smoother. That small adjustment made all the difference in the world.

Although the previous fuel line may have fit, for safeties sake it was determined that a new (and better looking) braided steel one be used. First step was to determine the proper length. With electrical tape wrapped tightly around the area to be cut, a cut-off wheel is used to cut the piece.

Demon offers 4 different vacuum step-up springs so top-end performance is easy to dial in, and multiple sizes of metering rods are available to make it easy to refine calibration. Multiple sizes of primary and secondary jets are also available to fine-tune the Street Demon for any elevation and application.

The new Street Demon has also been designed with ease of maintenance and installation in mind. Thanks to its single fuel inlet, running lines is easier and there’s less chance of fuel leakage from multiple connections. Speaking of less chance of leaking, the Street Demon has an integrated fuel bowl/main body design that places the gasket above the fuel level, so even that area has been refined with a modern design.

Power needs to be run to the electrical choke, so a “key-on” lead is found in the electrical loom. To better protect the lead wire, and to make the install look as clean as possible, a piece of braided sleeve is run over the lead wire. A “T-tap” style connection is used to splice the lead to the hot wire.

Removing the carburetor for changing jets has sometimes been a requirement with some carburetors. One of the cool features of the new Street Demon is that changing those jets and metering rods doesn’t require removing the carburetor and working on a bench.

The Street Demon’s fuel bowl design, which has a solid bottom, allows for no fuel to be above a gasket to virtually stop fuel leaks. -Scott Witmer

The design of the Demon allows you to remove the upper horn of the carburetor and to replace components with the carb still bolted to the intake, making track side adjustments quick and simple. Scott Witmer, Sales Manager for Demon, said, “The Street Demon’s fuel bowl design, which has a solid bottom, allows for no fuel to be above a gasket to virtually stop fuel leaks.”

In keeping with the easy-installation theme, the Demon also features an electric choke and has an integrated 700R4/200R4 kickdown cable mounting position, a Ford A/T kickdown position and an available TPS kit that works with 4L60 or 4L80 transmissions.

Available in 625 CFM and 750 CFM configurations, the Street Demon carburetors have all the cool visual design cues from the classic carbs but also uses the latest in computer aided design techniques and CNC manufacturing prowess. The result is a unit that not only looks good, but also works pretty darn well too.

16Driving Impressions

Carburetors don’t have the advantage of a bolt-on EFI system, so when it comes to starting up the car for the first time, it often puts the “ready-to-run” claim to the test. With the Street Demon, however, that test was passed with flying colors. The first start up required very little more than an idle adjustment. The first turn of the key provided all we needed to get the car running, and the idle was both smooth and consistent. The choke opened as promised within a couple minutes of warmup.


One slight adjustment to the GVS and the old girl had her legs back and was able to handle full throttle again.

Once the secondary air valve was adjusted, the carb was left alone. The owner is switching to an electric fuel pump, so further carb adjustments of the accelerator pump or jetting will be postponed until the desired fuel pressure is set at the regulator. Meanwhile, the car is logging additional miles to determine if any additional tuning adjustments need to be addressed at that time.

Granted, the car saw very little daylight or pavement in the past twenty-some years, but there’s something to be said about taking a carburetor out of a box, connecting the throttle and fuel lines, and driving the car away without altercation. It’s satisfying enough to install a carburetor on a regular driver and have it work right out of the box, but doing so on a car that’s seen little use in twenty years is something to boast about.

The new Street Demon carburetor can be found on the Demon Fuel Systems web site, where you can get even more information about this and other Demon carburetors and products. If you’re looking for a ready-to-run carburetor, the Street Demon is one that should be on your check list. Now there’s no excuse for not driving our classic car a little bit more.

About the author

Matt Emery

As an editor of Drive! and Classic Trucks magazines and staff editor on titles such as Dirt Sports, Off Road, and 4 Stroke Dirt Bikes, Matt Emery at one or another covered everything from the Baja 1000 to NASCAR, NHRA to the Bonneville salt flats and the Easter Jeep Safari to motocross. He now freelances to the Power Automedia online publications, among his many ventures.
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