1 With the 625 Demon carburetor,...
1 With the 625 Demon carburetor, it’s easy to get the feeling it is all new because you’ve never seen anything like it before. It is the smooth cast aluminum or automotive grade polymer structure, offering improved airflow. It is designed to replace any square bore or spread bore carburetor.
In this new age of electronic engine control, the humble carburetor seems lost to the old technical manuals, vintage swap meets, and dusty attics. So why an all-new performance carburetor from Demon when fuel injection has the obvious advantage? Because there are plenty of people who want the simplicity and crisp response of a performance carburetor, one they can tune themselves without a laptop or an exotic software program. They also want performance and fuel economy, both of which have been well thought out by Demon’s designer, Larry Tipton.
Larry has spent most of his life designing and engineering fuel systems. In fact, he began his career with Carter, a company many of us are familiar with, and has designed and engineered fuel systems for nearly half a century. Inspiration for the new 625 Demon came from the desire to conceive a carburetor unlike any before with real world improvements—dare we say the perfect carburetor?
When we asked Larry about his inspiration for an all-new carburetor design, he commented, “In carburetor design, as with any complex product, you need to keep the complete design in mind at all times. Nonetheless, one of the first challenges was to provide the new carburetor with instant throttle response. To this end we quickly settled on 13⁄8-inch primary throttle bores. To ensure that the primary venturii responds immediately and with authority, we decided to equip it with the strongest possible vacuum signal at the primary nozzles. So we adopted triple-stack venturii, a highly effective innovation that intensifies the vacuum signals and transfers them very quickly to the primary nozzles, giving great effect immediately at off-idle.”
Larry added, “Since its beginning, Demon Carburetion has specialized in racing carburetors, or for street-strip and muscle cars. Later, the Demon Six Shooter and Demon 98 carburetors and their attendant intake manifold systems were designed for small, specialized markets. But the company never had a carburetor specifically engineered for the popular performance markets, especially a lightweight aluminum, die-cast model. It was this niche that inspired the 625 Demon. We felt there was an opportunity to improve the appearance and function of the current AFB-style carburetors in the market today.”
The basic architecture of the Demon is loosely based on the Carter AFB/AVS carburetors with metering rods and jets along with a piston-style accelerator pump. This is where similarity ends. What makes the 625 Demon more advanced is long needed improvements—butterfly-style secondary transition air valve, spread bore design with “right sized” primary and secondary throttle bores, a main body designed to keep heat away from the fuel for improved performance, and a no-brainer adjustment system that makes engine tuning a snap.
When the engine is fired and at operating temperature, it is time for initial adjustments before the first road test. Adjustments depend largely on ambient temperature, humidity, and elevation. Idle mixture screws located on the front of the 625 Demon are strictly for idle air adjustment only. They have no effect on off-idle (open throttle) performance. At idle (approximately 600-800 rpm), you should have 17-22 inches of vacuum, depending upon the cam profile. One at a time, slowly turn the idle mixture screws clockwise until engine speed begins to falter. Then slowly back out counterclockwise until the idle is smooth. Do this on both sides. Then do it again one at a time until you have a smooth idle.
If you have a distributor with a vacuum advance, this is the time to check the vacuum advance function. When you check ignition timing, you’re looking for two things—centrifugal and vacuum advance function. As a rule, vacuum advance typically hands off to centrifugal advance as rpm increase. Total ignition timing at 3,500 rpm should be no more than 36 degrees BTDC. With the vacuum advance connected, goose the throttle and watch the timing mark. If advance is aggressive, you need less vacuum advance. If it is sluggish, you need more.
2 The Street Demon offers...
2 The Street Demon offers 13⁄8-inch throttle bores with triple-stack venturii. Two of the boosters are positioned over bores in a conventional manner. The third is buried in the throttle bores to provide greatly improved fuel emulsification for crisp throttle response. This system delivers power promptly when you need it.
3 Throughout more than a...
3 Throughout more than a century of carburetor evolution and design, there has always been the question of how to get prompt power without hesitation. This spring controlled air valve offers seamless transition from primaries to full on secondary when the accelerator is pressed. At the bottom of the secondary bores is the goggle valve known as “GVS” with more than twice the airflow of the primaries.
4 The Demon’s fuel bowl is...
4 The Demon’s fuel bowl is integral within the main carburetor body. This is the automotive grade polymer main body. This means no leaks because the fuel is contained below gasket level. And because synthetic polymers are the best they’ve ever been, you get solid reliability and performance no matter how toasty it gets underhood. Fuel is insulated from counterproductive engine heat.
5 The throttle linkage is...
5 The throttle linkage is compatible with just about anything out there. Early linear-style Mustang throttle linkages work with the 625 Demon as well as ’69-up cable linkages. This is also a good-looking piece with an easy to turn idle speed screw, which can be adjusted by hand.
6 In back, you’ll find constant...
6 In back, you’ll find constant vacuum ports, one for the PCV valve and the other for vacuum function accessories. There’s an additional plugged vacuum port as well for anything else you might have in mind, like the power brake booster. Throttled vacuum advance port is located in front close to the distributor.
7 What makes the 625 Demon...
7 What makes the 625 Demon unique is a common sense approach to throttle bore sizing. Primaries are smaller than usual for improved torque and greater fuel economy because you just don’t need large throttle bores for everyday driving. Secondaries are large where it counts—power making. They are engineered to open smoothly via a butterfly valve on top.
8 Demon provides high-grade...
8 Demon provides high-grade carburetor base gaskets along with a steel plate to help insulate from heat and provide a perfect surface that transitions from square bore to spread bore. This, coupled with the polymer main body, helps reduce fuel temperatures by as much as 20 percent.
9 After checking the base...
9 After checking the base gaskets, spacers, and plates for debris, we set the 625 Demon in place on our small-block intake manifold. Our thanks to Mustang Etc. for supplying a ’65 Mustang for installation.
10 Hand tighten the base...
10 Hand tighten the base nuts with a deep-well 1/2-inch socket or box end wrench. Snug in a criss-cross fashion. Do not overtighten. Check throttle for freedom of movement.
11 Our first generation Mustang...
11 Our first generation Mustang linear throttle linkage is installed next. To retain the throttle linkage, we’re using a circlip after drilling a small 1⁄32-inch hole in the end of the rod.
12 Always use a double throttle...
12 Always use a double throttle return spring (spring within a spring) for safety. One spring is not enough.
13 The ’65-’68 linear actuation...
13 The ’65-’68 linear actuation rod is adjustable to control accelerator pedal height. Have a helper hold the pedal to the floor to ascertain throttle position. With the pedal on the floor, you should have wide-open primary throttle.
14 For ’69-up cable throttle...
14 For ’69-up cable throttle linkages, there is a variety of cable attachment links included in the kit.
15 The Demon’s electric automatic...
15 The Demon’s electric automatic choke is fully adjustable. Adjust so the choke plate just closes, then observe how quickly it comes off during warm-up. Power comes from either the ignition switch or alternator “stator” post (“S” post), which is live only when the engine is running. Do not get power from the positive side of your ignition coil.
16 Although we’re using a...
16 Although we’re using a stainless steel braided fuel line hose, we prefer a steel hard line for reasons of safety. When budget is limited, use no less than braided stainless steel hose or high-pressure fuel injection hose and for no more than 12 inches between pump and your new Demon carburetor.
17 Idle mixture adjustment...
17 Idle mixture adjustment is performed at the primary throttle bores. Once the engine has stabilized at idle, each side needs adjustment. If you’re using a manifold vacuum gauge, you want the most vacuum possible at idle speed, which should be around 600-800 rpm. Vacuum should be 17-22 inches at idle speed. You want the carburetor at curb idle with a tweak of transfer slot visible with primary plates closed. Throttle position should be at idle, not fast idle on the choke circuit.
Original Autolite distributor vacuum advances are tuned with shims. Replacement advance units call for an Allen wrench. If you want a more aggressive rate of advance, subtract shims or turn the adjustment counterclockwise to reduce spring tension. If you want a slower rate of advance, add shims or turn the adjustment clockwise. Do this in baby steps. Add or subtract shims one at a time or turn the adjustment one-half turn. Then observe the rate of advance and how the engine responds.
During the road test, try different driving regiments. Try normal throttle through 1-2-3 upshifts, gently leaning into the throttle with each upshift. You should experience a steady application of torque through each upshift without hesitation or spark knock (rattling or pinging). When you punch the gas, response should be quick without hesitation. The secondaries should open smoothly without a nose over or flat spots. There should be no spark knock under hard acceleration. When you return to idle, deceleration should be smooth without surging or stalling.
Inspiration for the new 625 Demon came from the desire to conceive a carburetor unlike any before with real-world improvements—dare we say the perfect carburetor?
New 625 Demon carburetors are already factory adjusted and tuned for a positive installation and start-up experience. However, every application is different. With the throttle linkage connected, make sure the throttle opens fully and closes properly without binding or sticking. You don’t want too much spring tension nor too little. At wide-open throttle, throttle plates should be parallel to one another and 90 degrees to the baseplate. At idle, there should be no play in the accelerator pump linkage connecting rod. The accelerator pump should begin its function the instant the throttle opens. Otherwise, you’ll experience hesitation issues.
At idle, the closed throttle position can vary between carburetor sizes and applications. Observe the transfer slot with throttle closed. It should be approximately .020-inch, just barely visible with the throttle closed. Accelerator pump tuning should look like this with a .15-inch gap when adjusted at the center hole. For a more aggressive pump shot, move link to #1 hole. If the shot is too aggressive, go to #3 hole. The most stroke you want is .30-inch.
Accelerator pump tuning should look like this with a .15-inch gap when adjusted at the center hole. For a more aggressive pump shot, move link to #1 hole. If the shot is too aggressive, go to #3 hole. The most stroke you want is .30-inch.